This paper describes a mode decision method for the motion compensated 3-branch HD-MAC system. The conventional mode decision method consists of a priori and a posteriori decisions. The correctness of the mode decision has a high influence on the picture quality at HD-MAC receivers. The conventional scheme, however, does not provide a good decision result. So we propose a new method by which the modes of blocks can be determined more correctly. Our method is based on frame-exchange and motion compensated interpolation. The performance of our method is shown to be better than that of the conventional ones.