Growth mechanism of the (100) and (001) faces of ammonium sulfate was found to be different even at the same supersaturation. The growth rate of the (100) face was mostly proportional to the solution supersaturation. On the other hand, the (001) face began to grow after reaching a reasonably high supersaturation. The delay in growth of the (001) face at low supersaturation inferred to the strong interaction between crystal surface and solvent molecules was explained by molecular modeling. The calculated binding energy of a water molecule with each face showed that the water molecule is more strongly adsorbed on the (001) face than the (100) face. The symmetrical arrangement of ammonium sulfate molecules was shown to provide a good binding site in which a water molecule is more strongly adsorbed on the crystal surface and the relative affinity to the (001) face is also larger than that of the (100) face. Those calculation results strongly support the growth mechanism of the (001) face at low supersaturation. (C) 2012 Elsevier B.V All rights reserved.