Abstract: The Anderson-Prausnitz equation of state has been improved by modifying the attractive term of the generalized van der Waals canonical partition function. In this modification, the parameter characterizing the intermolecular attractive forces is considered to depend not only on temperature but also on density. The calculated results for pressure and fugacity of pure gases, and for saturated vapors and saturated liquids, by using this new equation of state are in good agreement with observed results. Extension to mixtures and multicomponent vapor-liquid equilibria will be presents in Part 2.