A new CFD simulation method is developed for a semi-planing boat in unsteady motion. The density function method for the free surface treatment and the moving-grid method for the moving body boundaries are applied. The motion of a boat is simultaneously solved by combining the equation of the motion of the boat with the flow computation based on the finite volume method. Simulations for two types of semi-planing boat are performed in the Froude number range from 0.5 to 1.0 and the simulation results are compared with the experimantal ones. It is shown that the CFD simulation method can simulate the flow around a semi-planing boat with sufficient degree of accuracy and it can be used as an effective tool for the development of hull form.