A method for calculating the added resistance of ships in regular oblique waves was developed, based upon Maruo's theory. It is assumed that the velocity potential of fluid motions around the ship's hull is obtained from the conception of strip method. In order to determine the strength, position and phase of the hydrodynamic singularities which represent a two-dimensional ship's section heaving, swaying and rolling, Isolated Singularity Method is used. Numerical calculations of the added resistance in regular oblique waves were performed using digital computer. It is shown that the calculated results of resistance increases coincide fairly well with the experimental results in regular oblique waves, and that the effect of lateral motions on the added resistance in oblique waves is almost negligible.