Abstract: The gas transport in nanotubes is discontinuous, which is similar to that of rarefied gas in macro-scopic conditions. Meanwhile, considering the large specific surface area of the nanotube, the contribution of ad-sorbed gas to transport cannot be ignored. Based on the equilibrium molecular dynamics and the grand canonicalMonte Carlo method, the diffusion and adsorption of different gas molecules in carbon nanotubes with different diam-eters were simulated. The diffusion coefficient is calculated by using the flux correlation function, and the surfacediffusion coefficient of gas is deduced by combining rarefied gas dynamics. The results show that the diffusion coeffi-cient of the gas in carbon nanotubes is several times or even an order of magnitude higher than that calculated byKnudsen formula, and the surface diffusion of adsorbed molecules is the reason for the rapid transport, and the sur-face diffusion coefficient increases with the surface coverage.