The effects of L-valine on the lateral growth of (011) and (120) surfaces of L-alanine are studied by combining molecular simulation and experimental studies. The growth of the (120) surface is significantly inhibited by-alanine, While the growth of the (011) surface is less affected. An intrinsic impurity effectiveness parameter is proposed by taking into consideration the Selectivity of impurities to adsorb On both step and terrace, which explains the different inhibition effects of an impurity on different surfaces.