An augmented van der Waals model of VLE of the pure compounds has been established by the molecular thermodynamic method

and an equation of calculating the enthalpy of vaporization at the normal boiling point is obtained from this model

which contains only a molecular size-dependent parameter.It is found that the parameter is directly proportional to the molar volume of liquid at the normal boiling point

and its values have the additivity of groups.The 45 group increments are regressed by 484 organic compounds.The calculation results of the enthalpy of vaporization for these compounds show that the average absolute deviation with the experimental values is 1.39% and is better than other methods published in the literatures.This method can be also used to calculate the enthalpies of vaporization at other temperatures.