In order to understand the load-carrying mechanism of thermal wedge, numerical results for a rectangular pad and a slider with parallel gaps under four types of surface boundary temperature conditions are presented. Two assumptions of rigid-solid and smooth-surface were used to exclude the effects of both thermal deformation and micro-asperity. The relation between thermal wedge and thermal boundary conditions is revealed. The load-carrying mechanism of parallel gaps is explained with the thermal wedge derived not only from the surface temperature difference (STD) as proposed by Cameron but also from the film temperature gradient (FTG) independent of STD. It is also pointed out that in numerical analysis, the very small viscosity–temperature coefficient would result in high oil temperature and therefore, the predicted loading capacity from thermal density wedge would be extremely enlarged.