The present study proposes a novel numerical method for fatigue life prediction under non-proportional loading. This method is employed for fatigue life estimation of different materials including 1045 Steel, 30CrNiMo8HH, Titanium TC4, extracted AZ31B Magnesium and Aluminum alloy 6061 under both proportional and non-proportional loadings. Basis of the method is developed in the framework of two numerical modifications. The first modification modifies fatigue damage parameters by correlating damages quantities of non-proportional loading to the proportional one. The second modification uses the same equation as the first one, but the corresponding damage coefficient is replaced by the additional hardening coefficient. In addition, these modifications are applied to fatigue damage parameters including maximum shear strain, SWT, Fatemi-Socie, and Babaei-Ghasemi model and also verified against experimental observations available in literature. Furthermore, the obtained results are discussed in details and also are compared to the non-modified findings. Moreover, the variation of the fatigue life prediction error is calculated for the aforementioned models. Finally, the results show considering and implementation of these modifications significantly improves the accuracy of the predicted fatigue lives for all the studied cases.