Due to necessity of increasing performance in new generations of the humanoid robots, in this paper, a novel power transmission mechanism to actuate the ankle joint of a humanoid robot is presented in order to increase the motion speed of SURENAIII humanoid robot. Also, the energy consumption of the proposed and the previous mechanisms are studied. In the proposed mechanism, the actuators of the ankle joint are located in the shank link. Then, a combined timing belt-pulley and a harmonic drive module are exploited for power transmission for the pitch joint. Also, the roll joint drive has employed a roller screw. In order to validate the design procedure, the simulation results of the robot are compared with the experimental data. The results reveal that the dynamic model is fairly matched to the real behavior of the robot. Also, the revolutionary genetic algorithm is employed to optimize the effective path planning parameters with respect to the minimum knee joint torque. This optimization procedure which is employed in robot walking on flat terrains consist of straight motion, ensures the robot's stability. As a result, the optimal path planning parameters for proposed mechanism are obtained in such a way that has decreased the actuating torques of lower-body of SURENAIII. Also, the proposed mechanism can achieve using lighter motors and getting the robot faster by means of mass reduction of foot.