Numerical simulations were carried out using a DEM-based model under a sliding fatigue environment to investigate the influences of differences of mechanical properties between intra-granular portions and inter-granular portions on local stress-strain fields and to determine the effect of gamma-irradiative degradation of UHMWPE components on delamination. The predicted stress fields in the gamma-irradiated UHMWPE component were highly nonuniform and were quite different from those predicted in the non-irradiated UHMWPE component. The stress distributions were intensely affected by the structural inhomogeneousness of networked grain boundaries. Marked stress concentrations were observed along subsurface grain boundaries in the gamma-irradiated UHMWPE. The gamma-irradiated UHMWPE showed a steep increase in the maximum equivalent stresses for an increase in the number of sliding. As the number of sliding increased, moreover, the region where the maximum equivalent stress was observed, moved deep into the specimen. These findings suggest that fatigue damage with plastic deformation proceeds mainly at a subsurface region in the gamma-irradiated UHMWPE and at a superficial region in the non-irradiated one. Therefore, gamma-irradiation is an important factor in accelerating fatigue crack initiation resulting in delamination.