Abstract: Sealing ability of faults is the key which controls the migration and accumulation of oil and gas along fault. In this paper, the authors reviewed and analyzed the fundamental ideas, in the prevailing evaluation of fault sealing property, and some new thinking about this work was made on the basis of the theory of effective stress. (1) The mechanical properties of a fault plane depends on the magnitude and direction of normal and shear stresses on that plane; compressive normal stress suggests that the fault plane is sealed: the nature of normal stress depends on the effective ground stress，which is otherwise closely related with formation pore pressure. (2)The prevailing evaluation of fault sealing property which completely neglected the influences of formation fluid pressure is not perfect; the fault sealing property has been changing for all the history in an instant, repeatful and eventful way. (3)Under deep burial, nearly all faults are generally sealed in relatively peaceful tectonic background. (4)In nature, there are at least two mechanisms which have repeatedly resulted in the natural fracturing of confined aquifers by high pressure: the first is “high pressure fracturing triggered by earthquakes”, i.e. the instant changes of geostress field during earthquakes can result in entremely high formation pressure which will further result in the opening of formerly sealed faults and subtle fractures; the other mechanism is “high pressure chain fracturing due to upward intrusion of deeply seated fluids.”