The basic principle of the sea surface measurement by active microwave remote sensing is the difference in microwave backscatter by the sea surface conditions. In order to develop more advanced measurement, it is necessary to understand the mechanism of microwave scattering at the sea surface in detail. This phenomenon consists of two physical processes. One is the formation of the sea surface by the composite action of wind, waves and currents, and the other is microwave scattering at that surface. This paper describes the analysis of the latter process by numerical simulation. The moment method that has been used for the antenna analysis is applied to the scattering problem at the irregular surface in recent years. In this research, the statistical characteristics of microwave scattering at the sea surface are analyzed by Monte Carlo simulation of the moment method. If only ripples exist at the sea surface, scattering coefficients calculated by the Bragg scattering theory and the physical optics (Kirchhoff approximation) are close to numerical results. Also in the case of the general sea surface, the influence of long gravity waves to the average backscattering coefficient is relatively small. This result assures that it is not necessary to take the influence of long gravity waves into account in the observation algorism of the wind.