There are the C.W.C and Towing Tank to the model-test equipments of the boat. A model testing of the high speed boat have a difficult in the performance verification because of very a small the scale-ratio of the ship-model and restricted by flow-velocity of the C.W.C and X-carriage velocity of the T.T. In general, the stepped hull boat is a high of fuel-efficiency because of the resistance reduction by a small wetted surface-area in correspond without stepped-hull boat. But It have a tendency to be bad the rolling performance by reduced stern wetted-area In this paper, the high speed stepped planning-boats with & without attached a stern body were performed to compare the effect of resistance and rolling performance by using sea model-test method.