The development and implementation of a control algorithm for the microprocessor-based stagnation pressure control system of the Penn State Supersonic Wind Tunnel Facility is reported. The gas dynamics and the controlvalve characteristics of this blowdown-type facility are nonlinearly related. A mathematical model was developed for the open-loop system characteristics and was linearized for the controller design. A single-input, single-output PI controller was chosen for this task because of its simplicity and availability. The resulting performance of the supersonic wind tunnel wds found to be quite good, with stagnation pressure variations typically held to within 1 to 2 percent.