Abstract: Engineering components are always in multiaxial and non-proportional stress states under complex service loading, and multiaxial fatigue is the primary failure mode during the long term vibration. In the present paper, the accuracy of multiaxial fatigue life estimation by the widely discussed Kandil-Brown-Miller (KBM) and FS model is investigated while the shortcoming of Fatemi-Socie (FS) parameter on fatigue life prediction of materials without additional cycle hardening is pointed out. Considering the dual influence of the additional cycle hardening and the rotation of principal stress/strain axes caused by non-proportional loading on multiaxial fatigue, which results in more fatigue damage, a new non-proportional influence factor is proposed, which is adopted for a modification to FS critical plane approach. Experimental results of five materials from tubular specimens under axial-torsional straining using sinusoidal wave forms in the literature are selected for the model verification. Comparing with FS parameter, the proposed critical plane damage parameter can significantly improve the accuracy of multiaxial fatigue lifetime prediction, especially for the materials without additional cycle hardening due to the non-proportionality of cycle loading.