The purpose of the study was to identify the mechanism of the black color formation in black garlic. Black garlics were homogenized,extracted with ethyl acetate and separated by vacuum silica gel column. The black petroleum ether-ethyl acetate-methanol( 99∶ 99∶ 1) elution fraction was identified by electrospray ionization mass spectrometry( ESI-MS) and high performance liquid chromatography( HPLC). Then the contents of the color material in different temperature and ripening period were determined by HPLC and the data were simulated. The results showed that the petroleum ether-ethyl acetate-methanol( 99∶ 99∶ 1) elution fraction mostly contained 5-HMF. This identification was done by ESI-MS which adds standard 5-HMF into samples. In addition,the content of 5-HMF increased with ripening period in different temperature. The formation reaction belonged to first order reaction through data fitting and the activation energy was 32. 11kJ/mo applying to arrhenius formula. This study confirmed that 5-HMF is one of the color materials. Meanwhile the reaction activation energy of 5-HMF was obtained,which provided the reference for the optimization of black garlic processing and the quality control of black garlics.