Understanding gas flow behavior and determining the surface diffusion coefficient are essential for investigating the gas flow mechanism and evaluating a shale formation. Although multiple gas transport mechanisms coexist in shale reservoirs with complex pore size distributions, the surface diffusion is very important and may even dominate the gas transport. Thus, the gas flow behaviors in shale are quite different with those predicted by conventional hydrodynamic methods. However, the experimental studies which systematically investigate the surface diffusion is very rare. Methane and Helium flow experiments were performed to understand the gas flow mechanism through micro/nano pores in shale and to accurately determine the diffusion coefficient. The experimental results such as gas flow dynamics curves are obtained at four different temperatures and five typical pressures. By analyzing the results and correlating with mathematical models, the diffusion coefficient for free helium transport and surface diffusion coefficients of both methane and helium are accurately calculated. We have divided the gas flow in shale into two main stages: free gas dominated flow and adsorbed gas dominated flow. This has been confirmed by comparing the gas flow behaviors of helium and methane in experiments. Besides, tests conducted under different temperatures show that higher temperature impedes the adsorbed gas amount but accelerates the dynamic gas flow process. Higher pressure not only increases the total gas flow but also enhances the gas flow behavior before equilibrium. In addition, by fitting the experimental results with a proper mathematical model, it is found that the surface diffusion coefficient increases with pressure. It is also observed that temperature has a more complex effect on the surface diffusion coefficient. Higher temperature results in lower concentration of gas molecular on the surface but more active thermal motion of gas. This observation is in accordance with our recent theoretical studies and can verify those assumptions experimentally. This study provides a straightforward method to experimentally determine the dynamic gas adsorption/ desorption-diffusion process and obtain the diffusion coefficient in shale. It enables more accurate predictions of field production and selection of future valuable candidates of exploitation for a given field.