Spatial resolution conversion method of image signal in image compression system based on motion compensation

A spatial resolution conversion method of a video signal in a video compression system based on motion compensation is provided to guarantee stability of resolution conversion, avoid implementation of a complex logic and greatly improve image quality. It is determined whether a frame of a video signal refers to another frame. Resolution of the video signal is converted by the first conversion method if the frame of the video signal does not refer to another frame. The resolution of the video signal is converted by one of the first and second conversion methods according to size of a motion vector if the frame of the video signal refers to another frame. The motion vector indicates variation of a block in the frame which refers to another frame with respect to a block in another frame to which the frame refers.