Abstract: Transient thermoelastohydrodynamic lubrication (TEHL) simulation and isothermal elastohydrodynamic lubrication(EHL) simulation were performed on the exhausting cam-tappet friction pair of an internal combustion engine.Results show that the center pressure,the thickness of the lubricant film and the friction coefficient,all calculated from two models,exhibit similar variation trend during a rotating cycle.The magnitude,however,presented significant difference.Results show that effect of temperature should not be neglected in researching the lubrication of cam-tappet pairs.