Three-dimensional nonlinear finite element (FEA) is developed to predict the experimental behaviour of concrete filled square steel tubular member. The FEA is conducted to determine moment carrying capacity at ultimate load for simple beam. The concrete-steel interface model is the important parameter affecting the result of FEA simulation. Based on the experimental result, there is a local buckling near the loading contact point. To investigate the local buckling phenomenon, concrete-steel interface model is studied by using contact analysis between concrete and steel elements, by using rigid bar element (rbe2 element) and by using interface elements. The geometrical non-linearity, material non-linearity, loading, boundary conditions is the same for all analysis models. To account for all of these properties, FEA model by means MSC Marc Mentat software is developed. The proposed model can predict the ultimate strength with difference only 5-30%. The collapse modes by FEA model are also compared. Based on the numerical analysis, it can be seen that the local buckling is clearly shown in the FEA model with the concrete and steel interface by using interface elements.