Abstract： The added resistance of ships in head waves is calculated under the assumption of weak nonlinearity in potential theory of ship motions. Maruo’s added resistance theory and the nonlinear ship motion theory are combined. The nonlinear ship motions are calculated using a time domain strip method, in which the FroudeKrylov force and hydrostatic force are nonlinear while the radiation force and diffraction force are still linear. Then the added resistance is obtained according to Maruo’s formula using the motion results. The computational results for S175 containership are used to validate the accuracy of the implemented methods and the results prove that the influence of wave steepness can be considered effectively. The influence of ship speed on the nonlinearity of added resistance is further investigated.