Based on the fatigue test results of SAE 1045 medium carbon steel and 40CrNiMo alloy steel, the axial and multi-axial fatigue life predication models are founded respectively. Relationship between the two kinds of models is studied. It is found that, so far as the two kinds materials are concerned, if the fatigue strength exponent and the fatigue ductility exponent are constant, while the fatigue strength coefficient times1.0937 and the fatigue ductility coefficient times 0.7512, the von Mises equivalent strain amplitude replaces the axial push-pull strain amplitude, then the modified axial fatigue life predication model is equivalent to the multi-axial fatigue life predication model. In the modified prediction model of multi-axial fatigue, the fact that the elastic line shifts up while the plastic line down indicates that given the same strain amplitude, the elastic strain component in uni-axial fatigue is larger than that in multi-axial fatigue while the plastic strain component in uni-axial fatigue is smaller than that in multi-axial fatigue. Therefore the revised axial fatigue life predication model can be used to predict the multi-axial fatigue life.