This study presents the semi active knee prosthesis with pneumatic cylinder performing under Finite State Control. For this purpose, the similarities between walks with the prosthesis and the normal gaits were examined morphologically. The finite state control of the prosthesis with pneumatic cylinder was developed under the basis of number of phases on the gait provided by direct measurement system. Associated gait phases and walking speeds were obtained from gyroscope signal and its derivatives, in a normalization algorithm. Stance and swing phase controlled prosthesis walking is the best control scenario for semi active-knee prosthesis with pneumatic cylinder than three phase and five phase control. In order to examine the performance of the prosthesis, the knee angle in degree has been considered as a comparison criterion. Associated performance tests were conducted both with image based motion measurement system and direct type motion measurement system.