Abstract: The original bromoxynil octanoate and its main impurities are identified by GC/MS. Original bromoxynil octanoate sample are dissolved in acetone, injecting the solution and separates on HP-5MS 30 m×0.25 μm×0.25 mm column coupled with GC. The peak information of significant impurities is captured by mass detector. The compounds are qualitatively confirmed by comparing with retention time and MS spectrum of the standards. Under above GC/MS conditions, all components are fully separated, impurity Ⅰ is measured to be m/z 158; impurity Ⅱ is m/z 317; impurity Ⅲ is m/z 434. The result indicates that impurity Ⅰ, Ⅱ and Ⅲ is methyl caprylate, 2,6-dibromo-4-cyanophenyl acetate, 2,6-dibromo-4-methoxycarbonyphenyl octanoate, respectively.