Methane diffusion is a very important gas-transport mechanism in shales. Meanwhile, one of the biggest differences between shale gas reservoirs and conventional gas reservoirs is a multiscale pore structure in the former. Two kinds of methane diffusion experiments are conducted in this work to measure methane diffusion coefficients in shale cores at supercritical conditions. Experimental results show that (1) a free molecular diffusion coefficient is averaged to be 1.214 x 10(-10) m(2)/s at reservoir conditions from the isobaric diffusion experiments; (2) however, the Knudsen diffusion, surface diffusion and configurational diffusion coefficients in the pressure decay experiments are more significant for shale gas development. It is presented that Knudsen diffusion and surface diffusion appear simultaneously as gas transports in matrix nanopores, the mean diffusion coefficients of which are 4.99 x 10(-14) m(2)/s for pores of a diameter smaller than 4 nm and 9.03 x 10(-9) m(2)/s for pores of a diameter bigger than 4 nm. The mean configurational diffusion coefficient for dissolved gas is calculated as 2.06 x 10(-22) m(2)/s. In addition, the four types of diffusion coefficients mentioned above are also theoretically calculated through their corresponding models to compare with the experimental results. Due to the measured methane diffusion coefficients corresponding to a wide range of pore sizes, a relationship between gas diffusion and pore size is obtained by the combination of theoretical and experimental results, and this can guide to further analyze the comprehensive diffusion behavior in shale gas development, as well as the relative contribution of each diffusion during different production stages. This work sheds light on the gas diffusion behavior over multiple pore sizes, which is beneficial for further quantitatively understanding shale gas transport in matrix during production.