Abstract: Pitting is one of the primary failure modes of gears resulting from crack initiation and propagation caused by the friction contact at the power transmission interface. The corresponding fatigue is essentially induced by multi-axial loadings, and the fatigue life is somehow related to the stress/strain states at a specific plane during the meshing process. The uniaxial fatigue methods are thus not applicable any longer to predict the lifetime. In this paper, a multi-axial fatigue life prediction model was established based on the critical plane method coupling with the gear meshing dynamics and elastohydrodynamic lubrication theory. The contact parameters at each time step in a meshing cycle, such as geometrical curvatures, sliding/rolling velocities and loadings, were determined based on the analysis of gear dynamic behaviors and then introduced into the lubrication analysis. A two-dimensional moving average filter was used to take the running-in effects on the surface roughness into account, which had proven to be capable of evaluating the morphology change during the running-in period while remaining the dominant characteristics of the initial morphology. Normally, the roughness peak height was decreased by 20%~50% by such a method, which had been previously proven to be reliable for research purposes on rough surfaces after the running-in process. The sectional morphology was introduced into the calculation of the lubrication film thickness, and a transient thermal elastohydrodynamic lubrication model in line contact was then developed by solving the governing equation system based on the non-normalized discretization method to calculate the surface pressure and tractions. The fast Fourier transform was employed to improve the calculation efficiency. The moving coordinate method based on the analysis of the motion distance in each time interval was used to determine the stress/strain at plane strain conditions in a fixed computational domain, and the near-surface elastic fields in a random plane were properly obtained based on the critical plane method by rotating the coordinate axis. Considering the fact that the gear pitting was caused by the initiation and propagation of mode I crack, the Smith-Watson-Topper method was then implemented to determine the fatigue life at each contact point. The point with the minimum life was the location where a crack probably first nucleates, and the corresponding plane was the one, along which the crack was likely to propagate. The prediction of gear pitting life was turned out to be in good consistence with that from the gear contact fatigue experiments, thus validating the present model. It was concluded that no friction was produced at the pitch point since the velocities of the meshed gears were the same, but the pitting was most likely to occur there due to the accumulating energy by the cyclic stress/strain. The results also indicated that the surface roughness had significant influences on the lubrication contact, resulting in large fluctuations of pressure, oil film thickness and temperature. The maximum von Mises stress was concentrated on the surface due to the roughness and thus resulted in crack formation from the surface. The lubrication would be squeezed into the crack as long as the crack was formed, which accelerated the crack propagation and rapidly leaded to gear failure. The model may provide ideas for the design optimization of the gearbox and offer insights into the gear surface damage due to the formation of opened cracks, and the developed method can be potentially extended to the life prediction of other power transmission components, such as the spline couplings and bearings.