This paper describes the mechanical design of a humanoid lower limb driven by series elastic actuator (SEA). The design is based upon the frame of ƯXA-90 humanoid, with the conventional rotation joints in spatial plane are replaced by SEA. A modular design of SEA is adopted for each specific joint (ankle, knee, and hip) so that different walking patterns can be applied to the humanoid for more flexible movement. By introducing the elastic element in the actuator, the humanoid can achieve higher fidelity of force control and better tolerance to shocks load during full cycle gaits. The experimental results validate the capability of the design and evaluate the performance of the actuators.