Abstract: A boundary element model is presented for the calculation of wave forces acting on arbitrarily shaped 3-D bodies due to wave diffraction. The fluid around the body is divided into inner region and outer region by a circular cylinder interface. The diffraction potential is described by an expansion of eigenfunctions in the outer region and determined in the inner region by solving the boundary integral equation using 1/r type Green function. Axisymmetric membrane elements as well as quadrilateral membrane elements are used to discretize the boundary surface of the inner region. The distributed singularity strength on an axisymmetric membrane element is approximated as a Fourier series of higher order to enhance solution accuracy. Numerical results are in good agreement with the linear theoretical solutions in the case of diffraction by a circular cylinder. Based upon the principle of spectrum analysis, the diffraction solutions of regular waves can be expanded to the multidirectional random wave environment to obtain the statistic character of wave forces.