Abstract: The modified van der Waalsmodel which has only one molecular size-dependent parameter is used to calculate the internal pressures of liquids.It is found that the model parameter has very good linear relation with the molar volume of liquids at normal boiling point.A group contribution method is established for the molar volume of liquids at normal boiling point.Therefore the internal pressures of various liquids can be predicted.On the basis of the generalized Gibbs free energy model for mixing of liquids,proposed by our previous work,a new solubility parameter had been defined,which is equal to the ratio of the cohesive energy density with the square root of internal pressure.This definition is more strict and its range of application is more exten sive than classical Hildebrand solubility parameter.In present work,the values of new solubility parameter for 170 solvents at 298.15 K are obtained.