To weaken the additional torque and noise caused by slot harmonics field, squirrel cage asynchronous motor with skewed rotor bars are widely used in small and medium cage induction motors. However, with skewed rotor, the distribution of the fundamental fields along the axial direction is non-uniformly, which may cause greater core saturation and higher iron losses . Furthermore, the inter-bar or the transverse currents are also produced by non-insulation between the rotor bars and lamination steels, which also lead to the excess loss significantly [2-4], especially for the small and medium motors. Therefore, it is necessary to eliminate the influence of the skewed rotors on the motor losses. In this aspect, a design method of non-skewed rotor bars for cage induction motors is presented , in which the asymmetrical straight rotor bars is adopted to weaken the slot harmonic field, at the same time, the excess loss caused by transverse currents can also be avoided. Therefore, it is helpful to design the small and medium size induction motors with premium efficiency (IE3 level). But the problem is how to design the parameters of the asymmetrical degree, air-gap length, slot combination and closed slot comprehensively.