Abstarct: Distribution functions have an important role in the extensive and development the theories of dense fluids. Most of thermodynamic properties of dense fluid can be evaluated with calculation of these functions. In this work have been tried to present a model for direct correlation function, DCF of simple monatomic fluids and interpret its behavior in different thermodynamic regions by using First-order perturbation theory and effective potential function. According presented model to in this work DCF by using the First order perturbation theory have been divided in to two contribution of core and tail. The core part of DCF has been considered as Percus-Yevick function for the DCF of hard sphere fluid however the molecular diameter of the hard sphere fluid depends on thermodynamic condition (density and temperature). Also, the tail part of DCF has been assumed as nonlinear and in the form of Mayer f- function type. The Lenard-Joens (12-6) potential has been used for the average effective pair potential along pair wise additive approximation for the molecular interactions. In order to found the DCF in different thermodynamic region have been calculated the effective intermolecular parameters in every temperature and density by solving the compressibility equation of state according to First order perturbation theory and pVT data. Hence to calculate the molecular diameter of fluid in the dense region for each isotherm which of observe slight change that with density disregarded of the molecular diameter dependence on density. Also well depth of such an effective pair potential calculates by solving integral equations for each temperature and density. The result have been shown that the presented model has a better agreement with the experimental data at high temperature and low density and also where increasing the density in the near critical point. We conclude that our model has shown more efficiency in those thermodynamic regions which used approximations in calculating intermolecular parameter with the pair additive approximation and PY approximation validate