If the buckling mode of the main side members can be kept in a good mode, it is possible to get a stead deceleration characteristic, and to reduce the weight. In this study, the authors proposed a new mode controlling approach to control the buckling mode of the side member. The results of the optimized design given by the new approach showed that not only can this new design method reduce the weight of the side member at the geometric design step, but also it can improve the basic characteristic of the energy absorbing behavior of side member.