This paper reports how mass distribution can affect the stability, efficiency and speed of a passive dynamic biped with upper body. To investigate the optimal mass distribution, we extend the simplest walking model with upper body, round feet and hip actuation. The mechanical parameters, including mass and the center of mass, are optimized by particle swarm optimization method. In addition, the effects of the hip torque on the walking characteristics are studied. Simulation results indicate that optimized mass distribution can improve walking characteristics of the passivity-based biped.