In this study, lubrication performance of mechanical seals was analyzed by a modified average flow model. In this new model, considering variabilities of the shear flow factor for the side leak effects and the pressure correction factor enable to reveal the effects of surface roughness on flat parallel sliding surfaces. As the results of this study, if the pressure correction factor was varied positively, as the Stribeck number increases, the film thickness and the film force ratio increase. On the other hand, if it was varied negatively, the film thickness and the film force ratio have the opposite trend. If the shear flow factor was varied negatively, the shear flow rate overcame the pressure flow and a flow from the low pressure side to the high pressure side occurred.