Demand for reduction in friction and improvement in wear resistance of moving parts propel exploration in frictional origin of amorphous carbon (a-C) film lubricating properties based on the interfacial states. Methane, as an ideal energy carrier and industrial raw material, is one of active gases. Consequently, the relations between the tribological behaviors of a-C film under methane atmosphere and load or interfacial states were discussed based on experimental and theoretical methods. It illustrated that, as the load increased, tribological system exhibited various interfacial shear strength at a load of zero and pressure dependence of the shear strength. The origin was that the distributions of adsorbates across the sliding interface which were similar to self-assembly layer changed with the load, leading to the source of energy dissipation. Therefore, the load dependence of the tribological properties of a-C films conformed to the Hertz elastic contact model by stages. [GRAPHICS] .