This paper describes about a study of a Surface-Mounted Permanent Magnet Synchronous Motor (SPMSM) with an optimal square wave current excitation which intends to improve an output torque. Although the SPMSM has a lot of harmonic flux in an air gap, usually a sinusoidal current drive uses only its fundamental component, or harmonics are reduced by a skew and so on, the output torque is not optimized. This paper investigates an optimal drive scheme in order to obtain the maximum output torque by using these harmonics. This scheme requires a full-pitch winding SPMSM and a square wave current excitation using a six-leg inverter to utilize all the components of the magnet flux in the air gap. We compare the output torque of the optimal harmonics current excitation and the sinusoidal wave current excitation under a condition of the equal current RMS.