Multi-length scale modeling is performed to (i) predict the carburized case depth of SAE8620 steel gears by solving the Fick's second law of diffusion,(ii) model the martensitic microstructure evolution in a grain inside the carburized case as well as to study the effect of stress cycling on retained austenite (RA)and martensite using a 3D phase-field model,(iii) simulate the effect of carburization and different RA contents on macroscale fatigue behavior of SAE8620 steel spur gear using the finite element method.The diffusion model predicts that the case depth increases with increasing heat treatment time and tem-perature.The phase-field simulations show that RA can transform to martensite during fatigue loading,where the extent of the transformation will depend on the type of stresses applied,i.e.stresses in a high stress regime or low stress regime of fatigue loading.Reverse transformation of martensite to austenite is also observed in low RA sample under high stress regime.The macroscale simulations show that the carburized case with high RA gives rise to better fatigue life compared to that with low RA.