Prosthetic legs have been used to restore function in the lower limbs lost due to amputation. Early designs including prosthetic legs with a passive joint or without any joint as well as the Energy Storing and Releasing (ESR) feet have shown deficiency in push-off torque, which results in asymmetric gait pattern, slower walking speed, and higher cost of transportation. Although powered prosthetic legs address the aforementioned problems, they suffer from lower energy efficiency, higher volume and weight. In this paper, a powered transtibial prosthesis using a Parallel Elastic Actuator (PEA) is proposed in order to generate the joint torque needed for walking with a lower-powered actuator for lighter and more compact design. A non-linear spring mechanism is proposed to generate the spring torque as needed. The implemented prosthetic leg is evaluated with three intact subjects. The experimental results shows that smaller torque is required for the motor with the spring mechanism. Therefore, less electrical power is consumed when the spring mechanism is used, which implies a lower-powered actuator is sufficient to generate the joint torque needed for walking.