<p>The oil film thickness of the piston ring at the sliding surface affects oil consumption, friction loss and wear of the ring and liner in internal combustion engines. It is important to know the oil film thickness on the piston ring sliding surface in order to improve the engine efficiency. Accurate measurement of oil film thickness on the piston rings is difficult because the oil film thickness is extremely thin. A modified method is proposed for measuring thin oil film. The authors have measured the oil film thickness on the piston skirt and land to contribute. The optical measurement system and analytic method are improved. The presented method present the oil film thickness on the sliding surface of the top ring from the oil film visualization images. The verification of the measured oil film thickness carried out by comparing with the data obtained from the numerical calculation. For evaluating the effect of the oil characteristics, 0W-20 oil and #8 oil were tested with different engine speeds and different crank angle where different ring loads. It was confirmed that the top ring oil film thickness became thick due to the high engine speed. A large load on the top ring reduces the oil film thickness. The oil film became thick when the oil viscosity was high. The effect of oil viscosity on the oil film thickness of the top ring was clarified. The effect of the viscosity index improver was also discussed. It is shown from the experimental results that the present measurement system for the top ring oil film thickness has enough accuracy.</p>