Why can't I open avi video in openCV?

A missing OpenCV's ffmpeg.dll does not generate any warnings/errors in OpenCV 2.3.1, and code fails silently. Make sure that you have proper opencv_ffmpeg*.dll in your path.

1)
Make sure the video file is actually in the same folder as the application (I'm assuming you've already tried this), otherwise specify the absolute path.

2)
If you're on Windows, you may need a codec pack to read the video file (e.g., K-Lite Codec Pack).

As Macmade suggested, AVI is merely a container which can house different audio, video, or even closed caption codecs. Also, here are Zeranoe's FFmpeg builds for Windows. You can get further information on your file's codec contents if you do the following:

ffmpeg -i guitarplaying.avi

You should see an output that looks like this:

ffmpeg version 0.8.7.git, Copyright (c) 2000-2011 the FFmpeg developers
  built on Dec  6 2011 09:20:43 with gcc 4.6.1
  configuration: --pkg-config=pkg-config --enable-gpl --enable-version3 --enable
-nonfree --enable-runtime-cpudetect --enable-memalign-hack --enable-postproc --a
rch=x86 --target-os=mingw32 --cross-prefix=i686-w64-mingw32- --prefix=/home/wluc
as/ffmpeg-cross/build/deploy --enable-libx264 --enable-libvpx --enable-zlib --en
able-bzlib --enable-libxvid --enable-libfaac --enable-libmp3lame --enable-libvor
bis --enable-libtheora --enable-libopenjpeg --enable-libfreetype
  libavutil    51. 30. 0 / 51. 30. 0
  libavcodec   53. 40. 0 / 53. 40. 0
  libavformat  53. 24. 0 / 53. 24. 0
  libavdevice  53.  4. 0 / 53.  4. 0
  libavfilter   2. 51. 0 /  2. 51. 0
  libswscale    2.  1. 0 /  2.  1. 0
  libpostproc  51.  2. 0 / 51.  2. 0
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '..\..\Videos\Sintel\sintel_trailer-720p
.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    creation_time   : 1970-01-01 00:00:00
    title           : Sintel Trailer
    artist          : Durian Open Movie Team
    encoder         : Lavf52.62.0
    copyright       : (c) copyright Blender Foundation | durian.blender.org
    description     : Trailer for the Sintel open movie project
  Duration: 00:00:52.20, start: 0.000000, bitrate: 1165 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1280x720,
 1033 kb/s, 24 fps, 24 tbr, 24 tbn, 48 tbc
    Metadata:
      creation_time   : 1970-01-01 00:00:00
      handler_name    : VideoHandler
    Stream #0:1(und): Audio: aac (mp4a / 0x6134706D), 48000 Hz, stereo, s16, 126
 kb/s
    Metadata:
      creation_time   : 1970-01-01 00:00:00
      handler_name    :

So, as you can see this .mp4 container has a H.264 video codec and an AAC audio codec.

May be you load the wrong lib file.Just like me，I get the same problem.But I can open AVI file in VC6.0 which use the opencv version 1.0.Finally，I find that I use the wrong lib.

In Debug Mode，but I use opencv_core220.lib and opencv_highgui220.lib .And I replace them with opencv_core220d.lib and opencv_highgui220d.lib .Now I can read it.