A powered lower limb prosthesis using a four-bar linkage mechanism was previously optimized, designed, and fabricated. Preliminary bench testing was conducted to demonstrate that it is capable of reproducing the normal ankle moment of a nonamputee. This paper focuses on the control aspect of this prosthesis. A finite state controller, which includes a state selector and a lower level controller, is proposed. Three sets of sensors and a sensing schematic will be used to determine the state of the device. Proportional-integral-derivative (PID) torque or position control will be used to realize the lower level control. Future testing will be done on lower limb amputees to prove the feasibility of this control schematic.