When a ship with a propeller advances in the sea with winds and waves, she usually loses in speed compared with the case in calm sea under the condition of constant output-power of the main engine. In this paper a method is presented that, in the case where the ship is moved almost straight in regular head waves, the propulsive performance is evaluated by using the calculated results of the pitching and heaving motions. Then, the relations among the ship speed, the rate of revolutions of the propeller and the delivered horse power are shown in numerical examples.