Genipin was obtained from hydrolysis of geniposide isolated from gardenia fruits with β-glucosidase. Reaction of genipin with glycine, alanine, histidine, lysine, phenylalanine and glutamate in aqueous buffer solution converted colorless starting materials to blue pigments. Effect of pH for the formation of blue pigments was tested using UV/Vis spectrophotometer. The optimum pH for the formation of blue pigments was 7.0. No pigment and trace amounts were formed at acidic (pH 3.0) and alkaline (pH 12.0) conditions, respectively. The amount and tincture of blue color were distinct with different amino acids. In contrast with lysine (λ_(max)=573 ㎚), glycine (λ_(max)=595 ㎚), phenylalanine (λ_(max)=602 ㎚) and alanine (λ_(max)=595 ㎚), the reaction of genipin with histidine (λ_(max)=601 ㎚) and glutamate (λ_(max)=601 ㎚) produced relatively small amounts of blue pigments. Rate constants for the formation of blue pigments from genipin with amino acids at various temperatures (60, 70, 80, 90℃, pH 7.0 phosphate buffer) were obtained. Rate constants of genipin with basic amino acids were larger than neutral or acidic amino acids. Arrhenius activation energies of the formation of blue pigments indicated that activation energy of glycine (E_A=9.8 ㎉/㏖) was especially lower than those of other amino acids (E_A=13.3∼15.4 ㎉/㏖).