Abstract: This paper presents an efficient method for sensitity analysis in shape optimization of continuum structures. According to mapping relations between design variables and element nodal coordinates, the explicit expression of sensitivity analysis is derived for 3-node elements. The difficulty of choose of step size of differece method is avioded. This enhances the accuracy and stability of result. This paper also studied the Modified Reduced Basis (MRB) and uses this method to calculate the structural responses and sensitivities. The example shows that MRB has high efficiency and satisfactory accuracy.