This paper proposes an optimum trajectory generation method of walking robot legs in consideration of specification of joint motors. It is based on the algorithm that generates the optimum trajectory using multiple curves. Four legged walking robot is assumed and the leg is composed by two links with joint motors at hip and knee. The optimum trajectory is obtained from minimizing the variation of torque from the rating torque, the maximum torque and the driving energy of the leg. The specification of the joint motors can be determined at the same time.