Experiments have been carried out on a clamped transducer system with electrical resistance loads and a transducer-horn system with water loads. The results show that the distributions of the vibration velocity amplitude of a Langevin-type vibration system change with loads and the wattmeter method implies certain theoretical errors. However, under light load conditions (< ρ VS) the error is small and the ratios of the amplitudes of two anti-nodes are almost equal under the condition of loads and without loads. The results also gave out the characteristics of power ultrasonic transducer systems, including frequency, load and power dependent characteristics of electric input impedance and electric-acoustic efficiencies. The results agree well with general linear acoustic theories. They may serve as guidelines on the wattmeter method even for applications of high-power ultrasonics, especially in sonochemistry.