In this paper, we derive an equation of state for liquid cesium based on a suggested potential function in accord to the characteristics of large attraction and soft repulsion at the asymptotes of interaction potentials. By considering the interaction of nearest adjacent atoms in dense fluid, the equation of state predicts that the isotherm (Z - 1)V-2 is linear function of (1/p), where Z is the compression factor, V is the molar volume, and p is the molar density. The linear parameters are identified as interaction coefficients related to attraction and repulsion and are used to evaluate the molecular parameters with interesting implications. The isotherm is intended to resolve the particular thermodynamic properties of alkali metals, which have been known for their unusual change of the nature of intermolecular force as the characteristic metal-nonmetal transition range is approached. When applied to liquid cesium, the isotherms persist linear over the whole liquid range including the metal-nonmetals transition range and at the critical temperature perfectly. The isotherm is equivalent to a virial (like) EOS for which the linear parameters of the isotherm form the corresponding second and third virial coefficients. The new potential function turns out to be an effective potential that includes not only a pair interaction but also many-body interactions, and therefore, it is not always comparable with pair potential.