Tension-compression fatigue tests under various mean stress conditions were conducted with round bar specimens of short glass fiber reinforced polybuthyleneterephthalate made by injection molding. Under cyclic loading with high mean stresses, the creep phenomenon became predominant and the ratcheting deformation increased with the number of cycles. This phenomenon is characteristic of plastics including short glass fiber reinforced plastics. The experimental data of the fatigue strength at the stress ratios above 0.7 were lower than the prediction based on the modified Goodman diagram. We propose to use the creep rupture strength, σc, instead of the tensile strength, σB, as the strength without mean stress and the parabolic equation for a constant life in the amplitude-mean stress (σa-σm) diagram. Our new design equation for the mean-stress effect on the fatigue strength on plastics is as follows: σa = σw – (σw / σc 2) σm 2, where σw is the fatigue strength at the stress ratio R=-1 and σa is the stress amplitude under a mean stress of σm. We also proposed a method to obtain the constant-life relation from limited experimental data.