Abstract: In present study, the nanoscale tribological behaviors of graphene on the diamond substrate surface were investigated using atomic force microscopy (AFM) in ambient air condition. Single-layer (SLG) and few-layer graphene (FLG) sheets were fabricated by thermal chemical vapor deposition and mechanical exfoliation method respectively and then transferred onto the surface of a polycrystalline diamond substrate, which was fabricated by microwave plasma chemical vapor deposition (MPCVD). The AFM results show that both SLG and FLG nanosheets remarkably reduced the nanoscale friction of the diamond substrate and exhibited extremely low coefficients of friction around 0.03 and 0.014, respectively. However, the friction of graphene on diamond surface was always higher than that on SiO₂/Si substrates due to their relatively weaker interaction on the diamond surface. Regarding to the influence of sliding velocity, under the normal loads of 0, 20 and 40 nN, the friction force evolution of graphene nanosheets on diamond substrates could be divided into three stages as the increasing sliding velocity. In the first stage, a roughly logarithmic increasing trend was observed for the sliding velocity up to 3 μm/s. The second stage saw a plateau as the sliding velocity increased from 3 μm/s to 10 μm/s. In the third stage, an increasing trend was dominated by the viscous damping term at the sliding velocity higher than 10 μm/s. Besides, the FLG sheets exhibited better wear resistance than the SLG sheets in a long-duration friction test under high normal load, which was supposed to be attributed to defects formed in SLG sheets and contaminants produced during the transfer process.