Abstract: Multiple input-multiple output (MIMO) system has becomes one of key techniques for developing B3G/4G systems. In this paper, a model of the MIMO system above a 3-D random rough surface is presented. Using Monte Carlo method, underlying random rough surface is realized. The Kirchhoff approximation (KA) is applied to numerical calculation of rough surface scattering for implementing multi-path propagation and the channel transfer matrix of MIMO system. It numerically shows the dependence of the MIMO system capacity upon physical parameters of the MIMO antenna arrays and underlying surface. It is found that when the inter-element separation of the antenna array is small, the underlying rough surface can significantly increase the MIMO capacity. However, when the separation is large, the MIMO capacity above the rough surface becomes lower.