The blocking of step motion on (100) and (001) faces of an L-isoleucine (L-Ile) crystal caused by the addition of an aqueous solution of L-norleucine (L-Nle) to the L-Ile solution was investigated using an atomic force microscope (AFM). The step motion was blocked in the presence of L-Nle. Crystal growth of L-Ile was inhibited along the a-, b-, and c-axes in the presence of L-Nle. Also, the blocking of step motion on the L-Ile (001) face of an L-Ile seed crystal was observed.