This paper describes the mechanical design for both a one and two degree of freedom above-knee (AK) prosthesis actuated by pneumatic artificial muscles. Powered prosthetics aim to improve the quality of life of the 50% of AK amputees who never regain the ability to walk. Pneumatic artificial muscle (PAM) provides great potential in prosthetics, since this type of actuator features a high power density and similar characteristics to human muscles. Currently, commercially available AK prosthetics are largely passive devices, and no research has been conducted on PAM actuators in AK prosthetics. In this thesis, the design requirements of an above knee prosthesis using PAM are discussed and a prototype one degree of freedom prosthesis with a PAM actuated knee joint is constructed. This prototype is then tested, and based on the results a new actuator is developed. This new actuator uses a flexible tendon and an elliptical pulley to improve torque, adding more functionality and increasing the maximum mass of a user by 25 kilograms. This actuator is also tested and compared to the initial prototype design. Finally, this new actuator is incorporated into the design of a two degree of freedom prosthesis with an actuated ankle as well as the knee joint.