Abstract: A molecular thermodynamic model for the calculation of ionic mean activity coefficients in electrolyte aqueous solution is developed by treating the ion - ion electrostatic interactions with mean spherical approximation (MSA) of all particles in the solution and other interactions with perturbation theory, respectively. Only the soft diameters of cations are correlated as the adjustable parameters in this model. The mean ionic activity coefficient data of 52 electrolyte (1-1,1-2,2-1 and 2-2 types) aqueous systems are correlated. Compared with other models, this model is rather simple and applicable and the calculation accuracy is satisfactory as well.