Understanding gas storage and transport mechanisms in shale is crucial for reservoir evaluation and gas production forecast. The shale matrix has a complex pore structure, with sizes ranging from nanometres to micrometres. Although diffusion plays a significant role in shale gas transport in the reservoir, systematic studies of gas diffusion in shale are rare. This paper studied the methane diffusion behaviour of shale based on pore structure, as well as the effects of sample particle size and water on gas adsorption and diffusion. The combined N-2 adsorption and SEM experimental results showed that the shale sample had a bimodal pore size distribution. The diffusion data were able to be described adequately by the bidisperse model, and the parameters were consistent with pore size distribution results obtained from the N-2 adsorption and SEM results. It was found that both Fickian diffusion and Knudsen diffusion play important roles in shale gas diffusion and they show different gas pressure dependence. Adsorption isotherm and calculated diffusivity showed little particle size dependence. However, gas adsorption and diffusivity were significantly reduced in moist samples, showing that water reduces gas storage capacity and transport rate in shale. (C) 2013 Elsevier Ltd. All rights reserved.