A study on the actual measured gas diffusion coefficient shows that calculating diffusion coefficient with the StokesˉEinstein equation is feasible.According to the mass balance principle，a comparison is carried out between the dischargeable gas（i.e.，the generated gasm inus residual gas）and themaximum diffused gas，amongwhich the smaller one is selected as the actual gas diffusion loss.The actual gas diffusion loss rises with an increase of organic matter abundance，hydrocarbonˉgeneration potential，source rock thickness and top burial depth.In a general geological condition，the calculated gas diffusion loss is several or even tens of times larger than the actual amount，causing an exaggeration of diffusion loss in the calculation of gas reserves.