An equation for calculating activity coefficient has been proposed on the basis of Group Solubility Parameter (GSP) model. The group solubility parameter is divided into four dimensions (contributions) and their values were determined by regression to fit the VLE of 172 binary systems selected from the literature. For these binary systems, the total average calculated deviations by the model were 3.52% in y(1) and 2.80% in P. The GSP model could be applied to predicting VLE of other binary or multi-component systems with good accuracy. Compared with other group contribution models, GSP model requires less adjustable parameters and their values can be easily determined.