A series of experiments on wave forces on a cylinder have been carried out when inertia component is dominant for a small size cylinder.The influence of nonlinear effect on the inertia component of wave forces on a cylinder is analyzed.The applicable range of nonlinear wave theories, such as Stokes and cnoidal wave theories, in calculating wave forces on a cylinder is discus9ed.A correction method is suggested for linear wave theory in calculating wave forces on a cylinder under the nonlinear condition.