The stability derivatives of a ship travelling in a following sea vary as functions of its longitudinal position in waves. As the primary cause of broaching-to phenomena, the loss of directional stability which a ship suffers when it is situated on the down slope of the following wave is pointed out. In order to examine the variation of directional stability in detail, to what extent the stability derivatives are affected by ship's position in waves should be more clarified both theoretically and experimentally. For this purpose, forced oscillation tests were carried out in following waves which was generated in an usual manner, that is to say, by the flap-type wave maker, and the stability derivatives were measured with both a rectangular flat plate which is a substitute for a ship, and a model with a realistic hull form. Simultaneously, stability derivatives of a rectangular flat plate with an aspect ratio equal to 0.2 were calculated by the numerical method advocated by Chapman. As the result of comparison between theoretical prediction and experimental results, it was shown that the numerical calculation performed for the flat plate can predict qualitatively the variation of stability derivatives of a ship due to its position in waves.