Abstract: A method and an apparatus for adaptively determining compression modes to compress images are provided. When compressing a current frame in image data, the invention inquires a motion class corresponding to a reference frame of the current frame, in which the motion class is determined according to motion information of the reference frame. Next, a threshold set corresponding to the motion class is obtained and used to determine a compression mode for compressing each of a plurality of macroblocks in the current frame. Finally, the determined compression modes are used to compress corresponding macroblocks in the current frame.