In recent years, studies about walking motion of biped robots have been developed rapidly. In many researches, biped robots are often treated as inverted pendulums since the structure of biped robots are very complicated. This simplification makes trajectory planning for a biped robot easy. An inverted pendulum model, however, gives some constraints to the robot. Walking motion using one kind of inverted pendulum has low robustness of walking. Hence, we propose a trajectory planning method with combination of two kinds of inverted pendulums in this paper. Trajectory planning of the swing leg for this new method is also proposed. With these, more stable and humanlike walking motion is achieved than that with one kind of inverted pendulum. The proposed method is more effective when a biped robot walks with lager range of strides.