Abstract: We calculated the contributions of the natural atomic orbital (NAO) of the molecular orbital S-Leu system under different implicit solvents by using the Gaussian natural bond orbital (NBO) analysis program. The results show that the implicit solvent methanol and chloroform have great influence on the molecular orbital of S-Leu system according to the contribution of NAO. The contribution of NAO of implicit solvent methanol is closer to the calculation result of the implicit solvent H₂O. The total contribution value of NAO of carboxyl fragment under implicit solvent methanol is closer to 
the calculation result of the implicit solvent H₂O. The effect of contribution of NAO of carboxyl fragment under implicit solvent chloroform is different from implicit solvents methanol and H₂O.

Key words: leucine molecule, molecular orbital component, natural atomic orbit (NAO), implicit solvent