The passive walker with knees can naturally execute the leg motion by the dynamics of legs under the gravity only. Circular arc foot is very important for the leg motion. In this paper, the dynamical effects of the arc foot, which can keep the knee joint of stance leg straight only by the stopper and can enhance the flexion of knee joint of swing leg, are discussed. Also, its geometrical negative effect is examined. Finally, the desired arc foot is designed and the actual effects are confirmed by a walking experiment.