This paper proposes a new maximum torque per Ampere (MTPA) control strategy for surface permanent magnet synchronous motors (SPMSMs) controlled by scalar control (v/f) method. In general, the scalar control method controls only the phase angle and the magnitude of the stator voltage vector. This approach can track the MTPA point without knowing the rotor position. The proposed method basically injects the high frequency voltage signal, and the input power calculated from the measured current and the command voltage is processed to generate the adjustment term of the voltage magnitude. Therefore, the phase angle of the current vector in the rotor reference frame can be changed. As a result, the MTPA for SPMSMs can be achieved in all speed-torque ranges and the system efficiency can be improved. In addition, this method is robust to the machine parameter errors such as the stator inductance. Simulation and experimental results are provided to verify the performance of this control strategy.