A method for object edge detection in a macroblock and a method for deciding quantization scaling factor are disclosed. This method calculates and compares the pixel means and means of absolutely difference of a plurality of pixel blocks in a macroblock to achieve the purpose of detecting if there is an object edge or flat area in a macroblock. In the meantime, the image structure of macroblock is analyzed and classified and its related messages are used for bit rate control, such that the visual effect of the compressed image can be appropriately enhanced.