Methods and devices are provided for down-sampling an image from H.264 to MPEG-4 at a high speed. The method for converting a first macro block having a first format into a second macro block having a second format includes: (a) determining a block mode of the second macro block and converting the first macro block into the second macro block with the determined block mode, when the number of intra macro blocks included in the first macro block satisfies a first condition; (b) determining a block mode of the second macro block and converting the first macro block into the second macro block with the determined block mode, when the number for inter macro blocks included in the first macro block satisfies a second condition; and (c) re-adjusting a motion vector of the second macro block, after the conversion of the first macro block into the second macro block is finished.