(Abstract) In this paper, we clarify the difference between the efficiency of IPMSM (interior permanent magnet synchronous motor) driven by a position sensor and that driven by sensorless control. First, we investigate the effect of control parameters on the estimation error. The efficiency of an experimental motor driven by the sensorless control using the suitable parameters is a little lower than that driven by the position sensor. Moreover, the efficiency under control of maximum of (torque/current) is higher than 0.8 % through 5.1 % than that under control of (id = 0).