3, 5-Dimethyl-4-hydroxy aniline hydrochloride was synthesized with a total yield of 75% from p-aminobenzene sulfonic acid via diazotization, condensation and hydrolysis reaction. 3, 5-Dimethyl-4-hydroxy phenyl-pentazole, which is stable at-50 ℃ was synthesized from 3, 5-dimethyl-4-aniline hydrochloride, and the yield was 66.7%. A small amount of pentazole anion was synthesized from 3, 5-dimethyl-4-hydroxy phenyl-pentazole by using methanol as solvent, ferrous salts as reductant, and m-chloro-peroxybenzoicacid as oxidant to break the C—N bond. Pentazole anion was characterized by MS and NMR. According to the isotope labeling experiments, nitrogen atoms in pentazole anion are in the same chemical environment, which means pentazole anion may possess aromaticity.