Abstract: The changes in hydrogen bond and valence bonds of methanol, ethanol, and propanol with temperature and pressure, the mixed state of methanol/oil, and the reaction process and mechanism of transesterification in sub/supercritical methanol in the temperature range of 15℃~300℃ and pressure range of 0.1MPa～25MPa were studied by using in situ attenuated total reflectance (ATR) infrared spectrum technology. The results showed that hydrogen bonds become weak for methanol, ethanol, and propanol in the temperature range of 15℃~250℃ at a pressure of over 14MPa, especially at the temperature of 75℃~225℃. But temperature almost had no influence on the vibrating peak of their -CH₃ bond. When temperature was over 225℃, the vibrating peaks of hydroxyl group of methanol got split, which could not be found for ethanol and propanol. Throughout the whole range of temperature and pressure, the obvious change in infrared spectrum of triolein was not found. At the same time, in situ ATR-FTIR also displayed that methanol and triglycerides were dissolved mutually to form a single phase when the temperature was higher than 185℃ at high pressure of 14MPa. For transesterification between methanol and triglycerides without catalyst at 14MPa, the initiative reaction temperature is about 220℃, which is near subcritical temperature of methanol. Therefore, the transesterification in supercritical methanol occurs in homogeneous state, and the new vibration form of C⁺…O^-…H⁺, which strengthens the electrophilicity and the nucleophilicity of small molecule alcohols, should be the main reason for accelerating supercritical transesterification.