Abstract: Based on a thorough study of the diffusion of natural gas through the cap rocks and its influential factors, the sealing of argilliaceous caprocks on natural gas in diffusive phase is analysed, suggesting that the sealing argillaceous cap rock on natural gas in a diffusion phase includes three forms, i. e a sealing (1) dependent of concentration, (2) of concentration and the quality of itself and (3) of the quality of itself.Considering the concentration difference of gas between a gas pool and its cap rock, the calculations of the diffusion rate of argillaceous cap rock and the diffusion rate of natural gas under unit concentration difference through the cap rock, etc. the authors inquire into the methods to study the three types of sealing capabiIity of argillaceous cap rock on natural gas in diffusion phase. The results of case study show that these methods are available, and can objectively reflect the actual sealing capability of argillaceous cap rocks.