Ultrasonic Power Transfer technology is beneficial to transmit power through the metal conductor, and does not produce electromagnetic radiation. As the maximum power transfer condition is still unknown at present, we propose a method to accurately determine maximum power transfer condition. By using electro-mechanical equivalent principle, we establish system overall model of ultrasonic power transfer system. On the basis, we build up system transmitter, receiver and propagation models and analyzed in detail. Eventually the system resonant frequency and optimal load for maximum power transmission is obtained. The results show the maximum power transfer condition is corresponding to the load matching point. The good agreement is reached between the theoretical and experimental results. This method is greatly useful for finding the load matching point to maximize output power of ultrasonic power transfer system.