Abstract: The irreducible water saturation is an important indicator of dividing oil & gas layer from water layer. The main factors in-fluencing the amount of irreducible water saturation have been analyzed on the basis of petrophysical laboratory testing results. Then the amount of irreducible water saturation can be calculated with the relationships between the affecting factors and the irreducible water saturation. After analyzing the characteristics of the irreducible water saturation of low permeable layer, the models of irreduc-ible water saturation with porosity, permeability, and the height of gas reservoirs have been established with an illustration of the low permeable layers in XX Basin. The applicability and accuracy of the models are checked by being applied in the actual situation and the result is good.