The solvent effects on both of the relative free energies of binding of Rb+ and K+ ion to 18-crown-6 and Deltalog K-s (the difference of stability constant of binding) have been investigated using a Monte Carlo simulation of statistical perturbation theory in several solvents. Comparing the relative free energies of binding of Rb+ and K+ ion to 18-crown-6 in H2O (TIP4P) and CH3OH in this study with the published works, there is a good agreement among the studies. The Born's function [i.e., (1 - 1/epsilon), where epsilon is the dielectric constant] of the solvents and the, differences in solvation dominate the differences in the relative free energies of binding and Deltalog K-s.