Abstract: The current study explored the hepatotoxicity among closed-ring genipin, open-ring tautomer of genipin and gardenia blue that generated from genipin and amino acid reaction using HepaRG cells to identify the material basis of genipin-induced hepatotoxicity in vitro. The effects of temperature, pH value and different kinds of amino acids on the chemical structure tautomerism between closed-ring and open-ring tautomer of genipin and the production of gardenia blue were investigated firstly, which aimed to explicit the conditions that could distinguish the closed-ring genipin and its open-ring tautomer, and the conditions generating gardenia blue, which were applied to prepare different kinds of gardenia blue; the CCK-8 kit was employed to analyze the hepatotoxicity of closed-ring genipin, open-ring tautomer of genipin and gardenia blue. From the results, it was found that, the structure transformation from close-ring to open-ring of genipin could be inhibited under the condition with acid environment; being essential groups to generate gardenia blue, the primary amino group and the open-ring tautomer of genipin reacting to generate the dihydropyridine ring was probably the key structure of gardenia blue; the structure characteristics existed apparent distinction at the reactive temperature of 37 ℃ and 80 ℃; compared to the culture condition with pH = 7.4, the concentration of genipin with close-ring in culture medium was significantly increased at pH = 5, but the cell viability did not decreased; the cell toxicity of gardenia blue was apparently lower than open-ring tautomer of genipin, and even some kinds of gardenia blue showed growth promoting effect on HepaRG cells. Here, it was suggested potentially that open-ring tautomer of genipin be the important material basis to induce hepatotoxicity, which could provide a cue and lay a foundation for the elucidation of the underlying mechanism of genipin-induced hepatotoxicity.