In this paper, the formation of MoDTC-derived tribofilm and effects of its characteristics on friction reduction was evaluated. To consider a transition of the tribofilm during friction, in situ observation of the tribofilm was conducted by using a reflectance spectroscopy, and then thickness and composition of the tribofilm was calculated. As a result, the rough specimen (\({R}_{a}=12.7 \text{n}\text{m}\)) showed a smaller friction coefficient and kept a larger volume fraction of MoS2 compared with the smooth specimen (\({R}_{a}=2.6 \text{n}\text{m}\)). In addition, we estimated a transition of a shear strength of the tribofilm formed on both specimens by using the in situ observed composition and contact state. From the estimation, it was indicated that the shear strength of the tribofilm determined the friction properties, and the direct contact of the asperities which was notable to the rough specimen encouraged a generation of a low-shear tribofilm.