The Author states in this paper following results out of his research including two actual ship experiments ; (1) The equation of motion of ships under steering can be reduced to a differential equation of 1 st stage as follows. T _s d θ/ dt +θ= K _s δ where T _s is a constant which represents the quick responsibility of ships, and K _s is a constant which represents the turning quality. Though, T _s and K _s will both vary with the curvature of the locus of ships, according to some experiments, the amount of variation is thought to be not so large. (2) The ratio K _s / T _s is a constant, i e, K _s / T _s = rudder turning moment/moment of inertia of the ship × rudder angle. and is easily calculated when the dimensions of the ship and rudder are given. So we can get K _s and T _s when either one is given. (3) It will be most reasonable to get Ks from data of steady turning experiment, i e, K _s =θ _s /δ where θ _s is the angular velocity of steady turning (4) T _s , K _s will both vary with the curvature of the locus of ship's C. G., i e. stronger the curvature, larger the both K _s , and T _s , in other wards, smaller the radius of turning, better the quick responsibility and worse the turning quality. But, the amount of variation is thought to be so small that we can get a close approximation of turning angle curve from initial stage to final stage using T _s and K _s got from the steady turning data.