It is well known that the strip theory is based on many assumptions. In those, three-dimensional effect, advance speed effect, viscosity effect and section-shape effect are investigated in this paper, and correction coefficients are given for them. The accuracy of the strip theory are examined comparing the responces, with and without correction, of the ship in regular head seas. The effect of viscosity and section-shape can be ignored, practically. But three-dimensional effect and advance speed effect are rather large over the wide range of λ/ L . To verify the practical accuracy of the strip theory, much should be investigated, such as nonlinear effect and restricted water effect.