Studies on the behavior of fluid in confined geometry are presently an area of research interest via experimental, theoretical and computer simulations. These researches shows that a fluid near a solid boundary becomes inhomogeneous and its properties differ essentially from the bulk case. Our results which have obtained from DFT have shown that this difference may be related to the three effects: the surface, the geometry of confinement and entropy effects. In this work, an analytic model for the prediction of the Helmholtz free energy of confined fluid in a nano slit has been presented with attention to the three reported effects and using the perturbation theory and pair wise approximation. The model is able to predict the liquid-vapor phase transition, compressibility factor and some regularities as well as bulk fluids, such as Zeno line and Rectilinear diameter Law. It has also been shown that the critical temperature increases with the size of the nano slit and finally it reaches the macroscopic value as the width of the nano slit approaches infinity. We have also shown that the slope of Zeno line increases with the size of the nano slit.
