The video formats supported by iMovie are: MP4, M4V, MOV, AVI, 3GP. If you got a unsupported video, such as FLV, WMV, MKV, VOB, MPEG, DAT, ASF etc, use a Mac video converter, which can convert video to all Mac players and devices, to convert it to iMovie acceptable format. Some .AVI videos cannot be imported into iMovie because of incompatible encoder, you also need to convert them. .3GP format is not recommended for the quality loss of the compression.
Info above is copied from this iMovie blog:
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