Abstract: The accurate determination of gas-production interface is crucial in the well location and perforation interval selection. With some preconditions, the conventional techniques used in determination of the interface neglect the effect of the gas-water transition zone. In this paper, a model to calculate irreducible water saturation from tested data of 729 core samples of four low-permeability gas fields was built. Logging data interpretation results from 12 wells showed that the new model could accurately determine the produced fluid type. Through classification and normalization of capillary forces in accordance with core permeability of 202 samples, correlation among heights of gas column, capillary forces and gas saturation can be constructed. The model then was used in calculation of gas production interface in formation S of A Gas Field in 2 exploration wells. The interface predicted from the model was then confirmed in practice and accuracy of the calculation model was also verified. Research results dsmonstrate that gas-production interface in low-permeability gas reservoirs can be determined accurately by using core data to optimize well location and perforation interval.