Pendulum test had been used for trial diagnoses of spastic human subjects. The angle θ of knee joint had been measured with a potentiometer and the first and second order derivatives of θ had been used for obtaining of angular velocity and angular acceleration, respectively. It was not easy however to hold the position of the potentiometer stable and even if the position slightly shifted, a large error might occur in the detected signal. Furthermore, the differential process trended to accompany a big noise. Then we developed a new method to measure the knee joint motions by the use of some accelerometer. The accelerometer could be attached to a lower leg easily and stably. If only one accelerometer was used however, both of knee joint angular velocity and knee joint angle calculated by integration might be accompanied with a large error, because it was under the influence of gravity. In order to overcome the difficulty, a measurement system for knee joint motions was constructed with a couple of accelerometers in this study. The accurate angular acceleration could be calculated as the difference between two output values of the accelerometers. The angular accelerations, angular velocities and angles in knee joint motions of normal subjects and spastic subjects were measured by the use of this system and it was shown that the error originated to the gravity decreased drastically comparing with the case of the only one accelerometer. This system would be useful for diagnosis of spastic patients by the use of pendulum test and for parameter decisions of the biomechanical and neural knee joint model.