Abstract: The tribological behavior of cylinder liner/piston ring was investigated with a stepwise heating mode. The oxidative degradation of the lubricant (SJ/CF 5W 30) and anti friction modifier (MoDTC) during friction and wear process was analyzed by means of Fourier transform infrared spectrometry (FT IR), laser Raman spectrometry, and X ray photoelectron spectroscopy, respectively. It has been observed that low coefficient of friction of the cylinder liner/piston ring pair can be maintained even at relatively high temperature under the lubrication of lubricant SJ/CF 5W 30 and of the lubricant containing MoDTC. Furthermore, valleys corresponding to a shape decrease of the friction coefficient appear on the friction coefficient curve, which is related to the oxidative degradation of the lubricant and the additive MoDTC during the friction process. When lubricated with the lubricant containing MoDTC, the valley at lower temperature is attributed to MoS 2 derived from the decomposition of MoDTC during friction process, while the valley at higher temperature is dependent on both the extent of pyrolyzing into graphitic carbon of the lubricant and the degradation of the additive MoDTC to MoS 2.