Based on Amonotons’ solid friction and the electrostatic force theory, the effect of grain boundary on the friction properties were studied by using atomic force microscope and Kelvin microscope for molybdenum disulfide and molybdenum diselenide nanosheets. The polycrystalline MoS2 and MoSe2 with monolayer were prepared by chemical vapor deposition, and the effect of grain boundary on the friction properties were measured in different regions (500 nm×500 nm) by friction microscopy module and Kelvin microscopy module. For the monolayer polycrystalline MoS2 and MoSe2, the roughness, contact potential and adhesive force in the obvious grain boundary area for the morphology map were larger than those in the non-obvious grain boundary region or the non-grain boundary region. The friction coefficient in the obvious grain boundary region was larger than that in other regions, and the friction coefficient in the non-obvious grain boundary region was larger than that in the non-grain boundary region. When the probe slided on the grain boundary, it was affected by both the ratchet effect and piezoelectric effect, and they leaded to the larger deformation and more energy dissipation. Therefore, there is higher friction in the obvious grain boundary region. For the increased friction in the non-obvious grain boundary region, it was only affected by piezoelectric effect. For the non-grain boundary regions, the friction were relatively small because the probe couldn’t affected by the ratchet effect and piezoelectric effect.