In this paper fully flooded and starved lubrication models considering surface roughness are developed for the piston ring and cylinder bore lubrication. The effect of oil available in the inlet region of a top ring is studied by comparing predicted oil film thickness, asperity contact friction and power loss under different oil supply conditions. The simulation results show that the proper oil supply is important in controlling the piston ring frictional power loss. The influences of lubricant viscosity and composite roughness on frictional power loss of piston ring are also discussed.