As the gas diffusion efficiency is key to understanding transport mechanisms in geomedia, this study is an attempt to understand how diffusion behavior are influenced with respect to granular size and water saturation to determine the relationship between diffusion coefficient (Dp, m2 s−1) and pore network (both geometry and connectivity). Diffusion chamber method with oxygen as the tracer gas were conducted on the six geologically different repacked sediments with six granular sizes. With decreasing of granular sizes, porosity measured, and diffusion coefficient calculated hardly fluctuates for the same sediment type, meaning the effect of particle size on pore network can be ignored for our six sediments. However, the obtained diffusion coefficient, varying in the magnitude of 10-7 to 10-6 m2 /s, could not be directly correlated to the changes in porosity, suggesting pore structure's complicated effect on gas transport. A decrease in diffusion coefficient after water saturation as observed in Grimsel Granodiorite with low porosity was attributed to the water blocking effect.