To analyze the chemical components and decomposition products in allicin extract of garlic, the chemical components screening and identification were made with HPLC-MS/MS method by full scan TIC MS, HPLC retention time, product MS spectra and chemical reference standards. The stability of the extract in water and alcoholic solutions was also investigated. There were five major components in allicin extract which were all identified as thiosulfinates. The extract was stable for at least 3 months when stored at -20 degrees C as water solution, but obvious decomposition was observed with the increase of alcoholic concentration. The decomposition products were also identified by HPLC-MS/MS.