In this paper, the principle of levitation force production for surface permanent magnet synchronous bearingless motors (SPMSBM) is introduced. The relationship of levitation force, magnet thickness and air-gap length in the SPMSBM was found based on the levitation force analytical model. The optimal thickness of permanent magnet to obtain the maximum controllable levitation force was studied and a function C is proposed to determine the optimal magnet thickness instead of finite element analysis (FEA). The result was confirmed by FEA.