Abstract: Diffusion is caused by concentration difference, while flow is by pressure difference. Concentration is a concept defined for solutions. Pure substances do not have the concept of concentration, thus there is no phenomenon of diffusion for them. If shale gas is viewed as a pure substance, there is no concept of concentration for it, in other words, diffusion phenomenon won抰 appear at all in it. If shale gas is viewed as a mixture, its composition won抰 be changed by any production process, so diffusion may not happen, too. Diffusion is defined for components in a solution, not for the solution, so "diffusion of shale gas" is not correct as a concept. The production of shale gas is actually the flow caused by pressure difference. During the production, reservoir pressure is usually monitored rather than concentration, hence the research on diffusion of shale gas has not any practical meaning