Many problems should be studied, when we compare measured ship motions with computed ones, such as three dimensional effect, advance speed effect, nonlinear effect and restricted water effect. Finite depth effect is investigated in this paper on the basis of Thin Ship Theory. There is little effect on hydrodynamic forces and on ship motions when water depth-ship length ratio h/L is greater than 1. but when h/L is less than 0.5, they are affected by the finite depth, and computed ship motions show a little decrease compared with those of infinite depth.