We have investigated the solvent effects on Deltalog K-s (the difference of stability constant of binding) and the relative free energies of binding of Cs+ and Rb+ ions to 18-crown-6, that is, the selectivity of Cs+ and Rb+ ions to 18-crown-6 using a Monte Carlo simulation of statistical perturbation theory (SPT) in diverse solvents. Comparing the stability constant (Deltalog K-s) of binding of Cs+ and Rb+ ions to 18-crown-6 in H2O (TIP3P) and CH3OH in this study with those in the experimental works, those of H2O (TIP3P) and CH3OH in this study are 0.80 and 1.57 and those of H2O and CH3OH in the experimental works are 0.57 and 0.61similar to1.24, respectively. 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 Cs+ and Rb+ ions to 18-crown-6. From the calculated coefficients of QSPR, we have noted that DN dominates the differences in relative solvation Gibbs free energies of Cs+ and Rb+ ions but Aj dominates the differences in the stability constant (Deltalog K-s) as well as the relative free energies of binding of Cs+ and Rb+ ions to 18-crown-6. Also, Born's function of the solvents, the polarities of the solvents, and the differences in solvation dominate the differences in the stability constant (Deltalog K-s) as well as the relative free energies of binding of Cs+ and Rb+ ions to 18-crown-6.