The solvent effects on the relative free energies of binding of K+ and Na+ ion to 18-crown-6 and Delta log K-s (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+ ion to 18-crown-6, in H2O (TIP3P) and CH3OH of this study with molecular dynamic simulation and experimental works, there is a good agreement among the studies.