diff --git a/doc/mruby3.1.md b/doc/mruby3.1.md
index 0c9ba8108..61c11803c 100644
--- a/doc/mruby3.1.md
+++ b/doc/mruby3.1.md
@@ -1,48 +1,91 @@
 # User visible changes in `mruby3.1` from `mruby3.0`
 
-# Build System
+# New Features
 
-## `build_config` directory
+## Core Language Features
 
-Several configurations for new platforms are added:
+### Keyword Arguments
 
-* `cross-mingw-winetest.rb`
-* `cross-mingw.rb`
-* `nintendo_switch.rb`
-* `serenity.rb`
+CRuby3.0 compatible keyword arguments are introduced.
+Keyword arguments are basically separated from ordinal arguments.
 
-Some new configurations for added for convenience.
+### Other Language Enhancement
 
-* `minimal`: minimal configuration
-* `host-f32`: compiles with `mrb_float` as 32 bit `float`
-* `host-nofloat`: compiles with no float configuration
+- Implement endless-def [Ruby:Feature#16746](https://bugs.ruby-lang.org/issues/16746)
+- Replace `R-assignment` by `single-line pattern matching` [Ruby:Feature#15921](https://bugs.ruby-lang.org/issues/15921)
+- Support squiggly heredocs. [#5246](https://github.com/mruby/mruby/pull/5246)
+- Hash value omission [Ruby:Feature#14579](https://bugs.ruby-lang.org/issues/14579)
 
-And `android_arm64-v8a.rb` was renamed to `android_arm64_v8a.rb` for consistency.
-
-# Configuration Options Changed
+## Configuration Options Changed
 
 Some configuration macros are available:
 
-* `MRB_WORDBOX_NO_FLOAT_TRUNCATE`: by default, float values are packed in the word if possible, but define this macro to allocate float values in the heap.
-* `MRB_USE_RO_DATA_P_ETEXT`: define this macro if `_etext` is available on your platform.
-* `MRB_NO_DEFAULT_RO_DATA_P`: define this macro to avoid using predefined `mrb_ro_data_p()` function
+- `MRB_WORDBOX_NO_FLOAT_TRUNCATE`: by default, float values are packed in the word if possible, but define this macro to allocate float values in the heap.
+- `MRB_USE_RO_DATA_P_ETEXT`: define this macro if `_etext` is available on your platform.
+- `MRB_NO_DEFAULT_RO_DATA_P`: define this macro to avoid using predefined `mrb_ro_data_p()` function
 
-# Language Changes
+---
 
-## Keyword Arguments
+# Updated Features
 
-CRuby3.0 compatible keyword arguments are introduced.
-Keyword arguments are basically separated from ordinal arguments
+## New build configurations
 
-## New Methods
+We have added several new build configurations in the `build_config` directory.
 
-* `Array#product`
-* `Array#repeated_combination`
-* `Array#repeated_permutation`
-* `Random.bytes`
-* `Random#bytes`
+- `cross-mingw-winetest.rb`
+- `cross-mingw.rb`
+- `nintendo_switch.rb`
+- `serenity.rb`
+- `minimal`: minimal configuration
+- `host-f32`: compiles with `mrb_float` as 32 bit `float`
+- `host-nofloat`: compiles with no float configuration
+- `android_arm64_v8a.rb`: renamed from `android_arm64-v8a.rb`
 
-# Internal Changes
+## Core Libraries
+
+### New Methods
+
+- `Array#product`
+- `Array#repeated_combination`
+- `Array#repeated_permutation`
+- `Kernel#__ENCODING__`
+- `Random.bytes`
+- `Random#bytes`
+- `String#center`
+
+### New Gem Enhancement
+
+- `mrbgems/mruby-pack` now supports `M` directive (Q encoding)
+- `mrbgems/mruby-pack` now supports `X` directive (back-up by bytes)
+- `mrbgems/mruby-pack` now supports `@` directive (absolute position)
+- `mrbgems/mruby-pack` now supports `w` directive (BER compression)
+
+## Tools
+
+- `mruby-config` now supports `--cc` and `--ld` options.
+- Remove `OP_` prefix from `mruby -v` code dump output.
+- Prohibit use of `OP_EXT{1,2,3}` by `mrbc` with `--no-ext-ops` option.
+
+## Features for mruby Developer
+
+- Add new specifier `c` to `mrb_get_args()` for receive Class/Module.
+
+---
+
+# Breaking Changes
+
+## Incompatibly Changed Methods
+
+- `Kernel#printf` (`mruby-sprintf`) Format specifiers `%a` and `%A` are removed.
+- `Kernel#puts` (`mruby-print`) Now expand Array arguments.
+
+## mruby VM and bytecode
+
+Due to improvements in the binary format, mruby binaries are no longer backward compatible.
+To run the mruby binaries on mruby 3.1, recompile with the mruby 3.1 `mrbc`.
+
+- Upgrade mruby VM version `RITE_VM_VER` to `0300` (means mruby 3.0 or after).
+- Upgrade mruby binary version `RITE_BINARY_FORMAT_VER` to `0300`.
 
 ## Reintroduced Instructions
 
@@ -53,18 +96,18 @@ Keyword arguments are basically separated from ordinal arguments
 
 `mruby3.1` removed following instructions.
 
-* `OP_LOADL16`
-* `OP_LOADSYM16`
-* `OP_STRING16`
-* `OP_LAMBDA16`
-* `OP_BLOCK16`
-* `OP_METHOD16`
-* `OP_EXEC16`
+- `OP_LOADL16`
+- `OP_LOADSYM16`
+- `OP_STRING16`
+- `OP_LAMBDA16`
+- `OP_BLOCK16`
+- `OP_METHOD16`
+- `OP_EXEC16`
 
 Those instructions are no longer needed by reintroduction of extension instructions.
 
-* `OP_SENDV`
-* `OP_SENDVB`
+- `OP_SENDV`
+- `OP_SENDVB`
 
 Those instructions for method calls with variable number of arguments are no longer needed. They are covered by `OP_SEND` instruction with `n=15`.
 
@@ -72,10 +115,11 @@ Those instructions for method calls with variable number of arguments are no lon
 
 `mruby3.1` introduces following new instructions.
 
-* `OP_GETIDX`: takes 2 operands `a[b]`
-* `OP_SETIDX`: takes 3 operands `a[b]=c`
-* `OP_SSEND`: takes 3 operands `a=self.b(c...)`; see `OP_SEND`
-* `OP_SSENDB`: takes 3 operands `a=self.b(c...){...}`; see `OP_SEND`
+- `OP_GETIDX`: takes 1 operands `R[a][a+1]`
+- `OP_SETIDX`: takes 1 operands `R[a][a+1]=R[a+2]`
+- `OP_SSEND`: takes 3 operands `a=self.b(c...)`; see `OP_SEND`
+- `OP_SSENDB`: takes 3 operands `a=self.b(c...){...}`; see `OP_SEND`
+- `OP_SYMBOL`: takes 2 operands `R[a] = intern(Pool[b])`
 
 ### `OP_GETIDX` and `OP_SETIDX`
 
@@ -85,6 +129,10 @@ Execute `obj[int]` and `obj[int] = value` respectively, where `obj` is `string|a
 
 They are similar to `OP_SEND` and `OP_SENDB` respectively. They initialize the `R[a]` by `self` first so that we can skip one `OP_LOADSELF` instruction for each call.
 
+### `OP_SYMBOL`
+
+Extracts the character string placed in the pool as a symbol.
+
 ## Changed Instructions
 
 ### `OP_SEND` and `OP_SENDB`
@@ -114,7 +162,7 @@ If the size of `mrb_float` and `mrb_int` are same, the last 2 bits in the `mrb_f
 
 Previous NaN boxing packs values in NaN representation, but pointer retrievals are far more frequent than floating point number references. So we add constant offset to NaN representation to clear higher bits of pointer representation. This representation is called "Favor Pointer" NaN Boxing.
 
-Also, previous NaN boxing limit the size of `mrb_int` to 4 bytes (32 bits) to fit in NaN values. Now we allocates integer values in the heap, if the value does not fit in the 32 bit range, just like we did in Word Boxing.
+Also, previous NaN boxing limit the size of `mrb_int` to 4 bytes (32 bits) to fit in NaN values. Now we allocate integer values in the heap, if the value does not fit in the 32 bit range, just like we did in Word Boxing.
 
 ## Constant Folding
 
@@ -146,3 +194,27 @@ GETIV R1 :@foo
 ## `String#hash` now use `FNV1a` algorithm
 
 For better and faster hash values.
+
+---
+
+# Major bug fixes
+
+- Fix infinite recursive call bugs in integer division [98799aa6](https://github.com/mruby/mruby/commit/98799aa6)
+- Fix to raise TypeError with super inside instance_eval / class_eval [#5476](https://github.com/mruby/mruby/pull/5476)
+- Fix to call `method_added` hooks on method definitions; [#2339](https://github.com/mruby/mruby/pull/2339)
+- Fix a potential buffer overflow in `time_zonename` [26340a88](https://github.com/mruby/mruby/commit/26340a88)
+- Fix `Module.instance_eval` bug [#5528](https://github.com/mruby/mruby/pull/5528)
+- Fix fix `M` packing bug [bfe2bd49](https://github.com/mruby/mruby/commit/bfe2bd49)
+- Fix a bug regarding attribute assignment with kargs [de2b4bd0](https://github.com/mruby/mruby/commit/de2b4bd0)
+- Fix SIGSEGV with mrbgems/mruby-method [#5580](https://github.com/mruby/mruby/pull/5580)
+- Fix print error before cleanup in `codegen_error()` [#5603](https://github.com/mruby/mruby/pull/5603)
+- Fix a bug in unpacking BER [#5611](https://github.com/mruby/mruby/pull/5611)
+- Fix a bug with numbered parameters as arguments [#5605](https://github.com/mruby/mruby/pull/5605)
+- Fix `mrb_ary_shift_m` initialization bug [27d1e013](https://github.com/mruby/mruby/commit/27d1e013)
+- Fix keyword argument with `super` [#5628](https://github.com/mruby/mruby/pull/5628)
+- Fix a bug with numbered parameters on toplevel [7e7f1b2f](https://github.com/mruby/mruby/commit/7e7f1b2f)
+- Fix keyword argument bug [#5632](https://github.com/mruby/mruby/issues/5632)
+- Fix multiple assignments in parameters [#5647](https://github.com/mruby/mruby/issues/5647)
+- Fix keyword parameters not passing through super [#5660](https://github.com/mruby/mruby/issues/5660)
+- Fix infinite loop from unclosed here-doc [#5676](https://github.com/mruby/mruby/issues/5676)
+- Fix negative integer division bug [#5678](https://github.com/mruby/mruby/issues/5678)