Ultra FLV joiner currently is the best FLV joiner, which can join FLV files perfectly. with this FLV joiner, you can also join FLV with other video formats together,such as join MPEG, AVI, RM, MP4 with FLV, this FLV file joiner also support join video files to VCD/SVCD/DVD image files, and then you burn them into your VCD/SVCD/DVD disk. with this FLV joiner you can even convert video files to other video formats you want. This FLV joiner is also a FLV converter, you can easily convert FLV to avi, mpeg, wmv with this FLV joiner.
Allok FLV Joiner is a powerful tool to join multiple FLV, MP4, RM, AVI, Divx, XviD, MPEG, ASF, MOV, 3GP files into one large file. It also supports join FLV video files to VCD/SVCD image and DVD-Video files, so you can burn them into one VCD/SVCD/DVD disc easily by using third-party buring tools. as the operation of Ultra FLV video joiner is simillar with Allok FLV video joiner, so I only list the guide of How to use Allok FLV video joiner
Guide: How to use Allok FLV Video Joiner Step 1: 
Free download FLV Joiner and install it on your computer.
Step 2:Click 
Button to select the video files you want to join
Step 3:Edit the source FLV video files, if you like.
Click
button to edit the selected file, you can set the start time and the end time, as the following picture :
Step 3, Sepcify the directory for output files by clicking the 'Change...' button, or just use the default directory (Note: after successful join, pls click Open button to find where the file stored, if u try to find the converted file via other way, you may can not see the joined file)
Step 5, Sepcify other settings for advanced user, you can set output quality, frame rate, TV type, Aspect Ratio, Add letterbox and so on.
Step 6, Click
Button to join the FLV video files.
After successful join FLV video files, the joined video files shall be stored in the folder you have previously selected.
Convinced? 
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UltraFLV joiner operation is quite similar as the guide above, just free download them and enjoy by youself.