A bit frustrating. The MP4 files from Canon EOS Rebel T6i are not well supported by iMovie, as you may encounter many audio and video problems.
"“I just purchase a Canon EOS Rebel T6i camcorder, and shoot some mp4 video clips, when I try to import the mp4 videos to iMovie for editing, problem comes, iMovie cannot recognized the files, so how can I solve this problem, or rather, how can I effortless import Canon T6i MP4 to iMovie on my Mac OS X? Thanks.”
Some people including you may be confused: iMovie says it supports MP4, but why I can't import EOS Rebel T6i MP4 to iMovie? Well, that's because iMovie prefers MPEG-4 and H.264 codecs instead of others while MP4 can contain various video codecs. So if your MP4 files are not accepted by iMovie, it's probably because of the video codec incompatibility.
To smoothly import and edit Canon T6i MP4 files in iMovie 11/10/09/08, the easy workaround is to convert MP4 to AIC MOV which is the best codecs with best results when working with iMovie. The best tool to help you get it done is Brorsoft Video Converter for Mac.
This converter can be used to convert any MP4 video to iMoviecompatible format ( AIC MOV) with fast conversion speed and high output quality. After the file conversion, you can edit the video as you like through iMovie. Meanwhile, this video convert tool can create files compatible with various non-linear editing systems like Final Cut Pro, iMovie, and Adobe Premiere Pro, Avid Media Composer, etc. If you are facing problems when editing Canon T6i MP4 video in iMovie on Windows, please turn to Brorsoft Video Converter. Download the software and this article will guide you through how to rewrap and transcode Canon Rebel T6i MP4 to AIC MOV for iMovie.
(Note: Click here for OS X 10.5) 
More Reading...
How to Convert Video_TS Folder to MP4
Guide: Convert TiVo recordings to Plex friendly video
How to import Nikon D5500 MOV files into EDIUS 5/6/7
How to rip Blu-ray to H.264 MP4/MKV/M4V/MOV with ease
How to Convert Canon G5X/G9X MP4 and Import MP4 to Final Cut Pro X/7/6
How to convert Canon EOS Rebel T6i MP4 to iMovie supported video
1. Import MP4 files
Click File > Add Video/Audio to add the MP4 movies from Canon EOS Rebel T6i you want to convert for iMovie. Or simply drag your videos to the left item bar for converting. Batch conversion is supported so you can add many files at a time to save the conversion time.
2. Choose iMovie compatible format
Click "Format" bar to choose the output format. Click and choose iMovie and Final Cut Express -> Apple InterMediate Codec (AIC) (*.mov). It’s the best format for editing Canon T6i video in iMovie 8/9/11.
Tip: Click Settings icon you can adjust video codec, bit rate, frame rate, sample rate, and audio channels, etc. To bear least quality loss, you may set A/V parameters according to the source MP4 file.
3. Start conversion
When all the settings are done, click Convert to start the conversion. Wait for the conversion to be done, you can import the converted video to iMovie 11 as well as iMovie 8/9/10 for editing without any trouble.
More Topics:
MP4 to Sony Vegas
MP4 to After Effects
MP4 to Avid Media Composer
MP4 to Windows Movie Maker
Tips for editing VOB files using Premiere Pro CC
Convert DVD to Samsung TV supported file formats
How to Bring Avid DNxHD MXF files into After Effects
Watch DVD VOB files on Surface Windows 8 RT Tablet
Edit XC10 MXF with Premiere Elements without problems
Editing Blackmagic 4K footage in Premiere Pro CC/CS6/CS5
Recover iPhone Text messages from iCloud backup selectively
How to batch convert AVI/MKV/VOB/MOV to Galaxy Tab S2
Tips fixing Canon C300 MK II Corrupt MXF files on Mac OS X
How can I "Import" Canon HF G20 .MTS (AVCHD) files to iMovie
