string fileName="e://abc.avi";

       string flv_file = System.IO.Path.ChangeExtension(fileName, ".flv");
　　　　System.Diagnostics.ProcessStartInfo FilestartInfo = new System.Diagnostics.ProcessStartInfo(ffmpeg);
       FilestartInfo.WindowStyle = System.Diagnostics.ProcessWindowStyle.Hidden;
       FilestartInfo.Arguments = " -i " + fileName + " -ab 56 -ar 22050 -qscale 6 -r 15 -s " + flvSize + " " + flv_file;

　　　　//转换
        if (!System.Diagnostics.Process.Start(FilestartInfo).HasExited)
       {
           Thread.Sleep(2000);
       }