Abstract: Tert-butylhydroquinone（TBHQ） as a food additive was researched by electron impact mass spectrometry (EI-MS). Based on the analysis of fragments appeared in the mass spectrum, as well as the structure characterizations of these compounds, some fragmentation rules were obtained. Results show that the EI-MS of TBHQ produces three compound-specific fragment ions: the m/z 166 molecular ion, the m/z 151 fragment ion which resulted from a methyl group loss of the molecular ion, and the m/z 123 fragment ion which resulted from a methyl group and the CO loss from m/z 166. At the same time, based on the related mass spectrum, the fragmentation mechanism of TBHQ was discussed.