Based on the model of hard--sphere molecules and weak molecular attraction the reaction equation of molecular aggregation is derived by statistical mechanical method in thispaper. And from the reaction equation the j-function describing phenomena of molecularaggregation is developed and then applied to modify the van der Waals equation of state.The modified equation of state is tested through the PVT data the calculated results showthat this equation is well suited to calculate the volume data of saturated vapors and liquidsfor substances, including some strongly polar, and associated compounds, such as methanol,ethanol, acetic acid and water etc.
