In this study, ship's maneuvering performance in waves is evaluated by using numerical computation. To this end, 3-DOF planar motions are considered, and modular type maneuvering model (MMG model) is applied. As the external force of the equation of motion, hull force, propulsion force, rudder force and wave drift force are adopted. In order to calculate wave drift force, seakeeping program which is based on higher-order Rankine panel method is used by considering wave frequency, wave heading, ship's forward speed, and ship's lateral speed. This wave drift force is pre-calculated with related variables, made into database, and used in time domain simulation. The developed simulation program is validated by comparing the computation results of turning test in regular waves with experimental data. Using this program, turning performance in irregular waves are evaluated and sensitivities for time signal of wave elevation are investigated.