We carried out optical resolution of DL-methionine-ethyl-ester (DL) by the diastereomer method using acethyl-D-methionine (D') as a resolving agent in order to elucidate the mechanism of purity drop in the course of optical resolution. We obtained about 90-95% of less soluble salt (D' . D) in the agitated system. However, the purity dropped to 85-90% in the non-agitated system. This purity drop in the non-agitated system was explained as follows with the help of calculated composition profiles of D' . D and D' . L in the stagnant film that developed near the D' . D crystal surface. D' . L is transported with the bulk flow induced by molecular diffusion of D' . D to the crystal-solution interface and accumulates on the crystal surface. This accumulation leads to nucleation of D' . L (impurity). Nucleation of D' . L causes the purity drop, especially in the non-agitated case. Although this type of nucleation can also occur in the agitated case, agitation enhances the deposition of required salt (D' . D), and hence the purity of agitated crystals is higher than that of non-agitated crystals.