Abstract: In the reservoir, the degree of fault vertical sealing has an important effect on the physical properties of crude oil. Based on the study of mechanism of sealing and its influencing factors, through comparative analysis of the normal pressure on the vertical section (δ_F) and the compressive strength of material in the facture zone (δ_C), a method for judging fault vertical sealing ability by using fault tightness index(I_FT) is established, which is defined as the ratio of δ_F to δ_C and is applied to study on the crude property difference of A Oilfield in Bohai Bay Basin. The result shows that I_FT is not the same for different faults and higher I_FT values would indicate improved fault vertical sealing property. In case that fault is broken to the base of the buried hill but the seabed is not communicated, heat from the deep layer migrates to the shallow layer, and forms a local high temperature environment in A Oilfield, inhibiting the degree of biodegradation and gives a good crude oil property. Due to the diversity of normal stress on fault surface and the filling substance of fault zone in different locations of the same fault, I_FT is constantly changing along fault strike. Owing to I_FT values of the main faults in the fault block area with a SW-NE decreasing trend, biodegradation of different levels results in the gradual rise of formation oil viscosity from southwest to northwest. The judging result accords with the crude property difference distribution of A Oilfield, which indicates that the method is feasible to judge fault vertical sealing property.