The solvent effects on the relative free energies of binding of K+ and Na+ ions to 12-crown-4 and Dlog Ks (the difference of stability constant of binding) have been investigated by a Monte Carlo simulation of statistical perturbation theory (SPT) in several solvents. Comparing the relative free energies of binding of K+ and Na+ ions to 12-crown-4, in CH3OH of this study with experimental works, there is a good agreement among the studies. We have reported here the quantitative solvent-polarity relationships (QSPR) studied on the solvent effects the relative free energies of binding of K+ and Na+ ions to 12-crown-4. We noted that DN(donor number) dominates the differences in relative solvation Gibbs free energies of K+ and Na+ ions and DN dominates the negative values in differences in the stability constant (Dlog Ks) as well as the relative free energies of binding of K+ and Na+ ions to 12-crown-4 and p* (Kamlet-Tafts solvatochromic parameters) dominates the positive values in differences in the stability constant (Dlog Ks) as well as the relative free energies of binding of K+ and Na+ ions to 12-crown-4.