A method of motion detection for a 3D comb filter video decoder is disclosed. In this method, a plurality of sampled data F.sub.mP.sub.x,y is obtained and temporarily stored after a composite video signal is sampled, wherein F.sub.mP.sub.x,y represents a sampled data of the y.sup.th pixel on the x.sup.th line of the m.sup.th frame inside the composite video signal, and m, x, y are positive integers greater than or equal to 0. Then, F.sub.m+1P.sub.x,y, F.sub.mP.sub.x,y, F.sub.m-1P.sub.x,y, and F.sub.m-2P.sub.x,y are used to determine a motion/still status of the composite video signal. Since the present invention performs the motion detection according to the composite video signal whose Y/C has not been separated yet, the present invention can accurately determine the motion level.