An above-knee prothesis helps an above-knee amputee to walk like a sound man. For this functions in swing phase, the damper of an above-knee prothesis must be controlled. In this study, we measure the normal gait and design the controller and observer for the above-knee prothesis. From the measurement, we obtained the joint angle data with the vision sensor and the joint angluar velocity data with the vibrationg gyroscopes. For the control of the damper, we use this measurement data of normal gait. And the designed controller is consisted of three parts. The first is an estimation of a gait period with the output of the thigh gyro. The next is a generation of a desired knee angle trajectory based on the esimated gait period. The last is the computed torque method for the determination of damping coefficients. The observer for the above-knee prothesis is designed using the extended Kalman Filter. This observer based controller has a good performance.