The effect of hydrophobic amino acids as additives on the crystallization kinetics of a L-glutamic acid (L-Glu) crystals and the inclusion behaviors of these additives in L-Glu crystals were investigated. Four amino acids L-valise (L-Val), L-leucine (L-Leu), L-isoleucine (L-Ile) and L-norleucine (L-Nle) were used as additives. The crystallization rate of L-glutamic acid (L-Glu) was retarded by these additives and it was found that the effect of L-Val is larger than L-Leu isomers, and in L-Leu isomers, the retardation effect is the largest for L-Ile and the smallest for L-Nle. In every case, additive molecules are included in L-Glu crystal and the maximum molar ratio of additive to L-Glu is about 2.5 x 10(-2). L-Val with the smallest molecule size is most included in L-Glu crystals and the L-Leu isomers take similar R value. It was suggested that the effect of L-Val on the crystallization rate is due to the high adsorption density and the readiness of the inclusion of L-Val molecule in L-Glu crystals rather than the steric hindrance of the substituted group. In L-Leu isomers, the methyl group at the beta carbon seems to be effective in inhibiting the growth of L-Glu. The amount of additive included in crystals could be expressed well by isotherms of either Langmuir or Freundlich type.