Abstract: An approach to perturbation theory of fluids (including liquids and dense gases) is described. Two equations of state for hard particle reference fluid are series representations. These are obtained from a new radial distribution function For hard spheres, the three-term truncated form of the equations are the same as the pressure and the compressibility equation of state given by the Precus-Yevick theory. The molecular potential of real fluid is approximated with a number of square-well nonspherical potentials and the higher-order perturbation terms are obtained by using generalized Barker-Henderson approximation. A formula for the thermodynamic properties of multi-square-weii nonspherical potential hard particle perturbation is being derived.