A new concept of differential phase has been proposed. The flux of molecules between two phases, rather than the fugacity, is used to formulate the phase equilibrium. A double-resistance physical model is presented for modelling the flow of molecules through the interface of two phases. By equating the equilibrium between differential phase and bulk phase, a generalized sequential-analytic method for determining the radial distribution function (RDF) is developed. The proposed method is capable of giving RDF values close to those obtained by molecular simulation. It has been shown that when coupled with the Lennard-Jones potential model, the pressure-volume-temperature (PVT) relation of pure fluids with molecular size up to n-hexane can be predicted satisfactorily in wide ranges of temperature and pressure.