OBJECTIVE：To establish a method for the determination of related substances in dibasol hydrochloride raw materials and tablets ， and to predict the maximum unknown impurity ’s structure. METHODS ： The related substances （o-phenylenediamine，phenylacetic acid ）in dibasol hydrochloride raw materials and tablets were determined by HPLC. The determination was performed on Kromasil C 18 column with mobile phase consisted of mobile phase-methanol-glacial acetic acid-triethylamine（45 ∶ 55 ∶ 0.5 ∶ 0.5，V/V/V/V）at the flow rate of 1.0 mL/min. The detection wavelength was set at 220 nm，and column temperature was 30 ℃. Sample size was 10 μL. UPLC-TOF-MS，1H-NMR and 13C-NMR were used for structure prediction. The determination was performed on Waters Acquity UPLC BEH C 18 column with mobile phase consisted of water-methanol （45∶55， V/V）at the flow rate of 0.2 mL/min. The column temperature was 30 ℃，and sample size was 1 μL. The ion source was electrospray ion source . The scanning mode was negative ion scanning mode. The first-order mass spectrum scanning range was m/z 100-800，the capillary voltage was 3 000 V，the source temperature was 100 ℃，the desolvent gas was nitrogen ，and the solvent free gas flow rate was 600 L/h. The flow rate of the conical orifice was 50 L/h. RESULTS： The linear range of o-phenylenediamine，phenylacetic acid and dibasol hydrochlo- ride were 0.427-4.27 μg/mL（r＝0.998 9），0.403-4.03 μg/mL（r＝ 0.998 9）and 0.82-8.20 μg/mL（r＝0.999 9），respec-tively. The limits of quantitation were 0.042 7，0.134 3，0.088 7 μg/mL. The limits of detection were 0.021 4，0.067 1，0.044 3 μ g/mL. RSDs of precision ，stability，reproducibility and durability tests were all less than 2％. The average recoveries were 98.31％- 99.78％-102.23％ for phenylacetic acid （RSD＝0.70％，n＝9）. No o-phenylenediamine was detected in 6 batches of dibazol hydrochloride raw materials ；the contents of phenylacetic acid · were 0-0.04％ ；the contents of maximum unknown impurity were 0.05％ -0.25％ ；total contents of unknown impurity were 0.05％-0.31％. In 77 batches of Dibasol hydrochloride tablets ，the contents of o-phenylenediamine were 0-0.11％，the contents of phenylacetic acid were 0-0.03％；the contents of maximum unknown impurity were 0.06％-0.51％；total contents of unknown impurity were 0.10％-0.62％. It was speculated that maximum unknown impurity was 2-（hydroxyphenylmethyl）benzimidazole （hydrobenzde）. CONCLUSIONS ：Established method is rapid ，accurate and specific ，and can be used for the determination of related substances in dibasol hydrochloride raw materials and tablets. The maximum unknown impurity may be benzimidazoles.