A numerical method to analyze the efficiency of OWC wave energy device with a lateral opening will be developed in this paper. The computational domain is divided into two parts: outside and inside the device. The linearized wave scattering theory is employed in the outer domain. In the inner domain, the pressure over the internal free surface is considered as being proportional to the square of the velocity on it. Iteration is then used to obtain the solution to velocity of fluid. This enables one to predict the power extracted by the device and to investigate the influence of some parameters. The results obtained are compared with the experimental data and fair good agreement has been found.