The bipedal humanoid robot WABIAN-2R is developed to simulate human locomotion. Performing a walking motion requires a high torque at the ankle joint. WABIAN-2R consists of harmonic gears in its driveline system which increases the weight of each leg and respectively decreases the energy economy. Therefore, a new idea is proposed and developed in this paper through computer simulation to modify the design of the ankle joint by adding a spring mechanism instead of high gear ratio transmission. The spring stiffness could be controlled by twisting the joint to set the required torque. This helps reduce the energy consumed while walking; by storing and returning part of the energy. Research presented in this paper proposes an important step toward developing the passive dynamics walking into the advanced complex humanoid robots.