This study presents a design technique of an efficient biped walking robot on level ground with a simple mechanism based on passive-dynamic walking. A torso is used to generate active power replacing gravity used in passive walk. Swing-leg control is introduced to create a steady gait. Numerical simulations show that a biped robot with knees and a torso can walk efficiently on level ground. When we choose an appropriate parameter of the swing-leg control, the biped robot can walk stably over a wide range of speed. Furthermore, the walking performance of the robot increases with the increase of the radius of circular feet.