Abstract: The effective diameter of a cation is introduced into the mean spherical approximation(MSA)method. The ionic strength dependence of effective diameter is attributed to the effect of solvation mainly. The effective diameter parameters of the cations have been obtained from regression of the mean ionic activity coefficient data. The range of concentration extends up to saturation when data are available. The modified model has been used to correlate the mean ionic activity coefficients for 85 single electrolyte solutions. The calculated results of the modified mean spherical approximation, original MSA model and Pitzer model are compared with the experimental data. It shows that the modified mean spherical approximation model gives the best representation. The modified mean spherical approximation model has been used to predict the mean ionic activity coefficients at other temperatures by using the parameters obtained at 298.15K. The calculated data show that the temperature dependence of the parameters in this model is smaller than that in the other four models (Pitzer, Bromley, Meissner and Chen).