Compliant offshore structures such as spars,tension leg platforms(TLPs) and semi-submersibles have been dramatically improved in recent years due to their capability for deep water operation. Waves are the most important environmental phenomenon affecting these offshore structures. Estimation of wave forces is vital in offshore structure design. For large compliant offshore platforms,Morrison’s equation is not valid anymore and usually diffraction theory is used. In this research,by using the finite difference method,a detailed analysis of the first-order diffraction of monochromatic waves on a large cylinder as a structural element is performed to solve the radiation and diffraction potentials. The results showed that the developed model is a reliable tool to estimate the wave forces and hydrodynamic coefficients on large structure elements when wave diffraction and radiation are considered. Compliant offshore structures such as spars, tension leg platforms (TLPs) and semi-submersibles have been dramatically improved in recent years due to their capability for deep offshore operations. Estimation of wave forces is For large compliant offshore platforms, Morrison’s equation is not valid anymore and usually diffraction theories is used. In this research, by using the finite difference method, a detailed analysis of the first-order diffraction of monochromatic waves on a large cylinder as a structural element is subjected to solve the radiation and diffraction potentials. The results showed that the developed model is a reliable tool to estimate the wave forces and hydrodynamic coefficients on large structure elements when wave diffraction and radiation are considered.