We explored the mutual incorporation tendency of branched chain amino acids (L-leucine, L-isoleucine. L-valine) and isomorphic amino acids in cooling crystallization conducted in water solvent. lit most cases, a guest amino acid (an impurity amino acid) whose side chain is longer than that of a host amino acid (a purified amino acid) was incorporated easily in a host amino acid. In this case, a solid solution was its formed, and the e-axis of a host crystal structure was extended. Also, the crystal growth was inhibited. On the other hand, when a guest amino acid whose side chain is shorter than that of a host amino acid was incorporated in a host, the lattice length and the crystal morphology of a host amino acid was the same as that of control. Front these results, a mutual incorporation model was proposed.