The first-ever application of a hydrogen-free diamond-like carbon (H free DLC) coating to a mass-production gasoline engine reduces friction between the cylinder bore and piston by 18%. It is explained the reducing friction effect of H free DLC by oil film thickness of piston ring. When H-free DLC is applied to top ring, friction is over 10% reduced. Because low friction coating is effective in a thin oil film at a large area near top dead center (TDC). It is also found that the friction reduction effect is enhanced when low viscosity engine oil is used. DLC as a low friction coating is more effective in a thinner oil film. In this study, the adhesion strength of H-free DLC coating on piston rings is improved because of providing a smooth substrate, the new washing process and the optimized coating characteristics. Adhesion strength is verified by test results using actual parts considered to contribute to piston ring load. Piston rings with low friction and high reliability are developed.