Manufacturing tolerances are inevitable in nature. For the bearings used in internal combustion engines, the manufacturing tolerances of roundness, which can be very close to the bearing radial clearance on scale, will affect the lubrication of the bearings. However, there is insufficient understanding of this mechanism. The current study aims to find out the effects of the amplitude and the phase of journal roundness in the shape of ellipse on the lubrication of engine bearings, and interaction impacts of these two factors are also studied. First the elastohydrodynamic (EHD) theory is applied to model the bearing as the roundness is only of a few microns and the EHD model can be more precise by considering the elastic deformation of the journal and the bearing bush. The analysis of the DOE results shows that the roundness amplitude decides the degree of the variation of the bearing friction power loss. It is also concluded that even with the same roundness geometry and under the same condition of load, the bearings may be at different lubrication conditions due to different roundness phases. However, in addition to worsen the lubrication conditions of bearing at times, some degree of roundness amplitude can be beneficial to the lubrication at certain roundness phase.