Abstract: To establish an HPLC method to determine osimertinib mesylate, Agilent ZORBAX Eclipse Plus C₁₈ column(4. 6 mm × 250 mm, 5 μm)was used with a mobile phase consisting of methanol-buffer solution(20 mmol/L NaH₂PO₄, pH 3. 0 adjusted with 85% H₃PO₄)(50 ∶50)at the flow rate of 1. 0 mL/min. The detection wavelength was 210 nm, and the column temperature was kept at 35 ℃. The calibration curve was liner over the range from 50% to 150% of determination concentration(0. 201 1- 0. 603 2 mg/mL, r=0. 999 9). The limit of quantitation(LOQ)and limit of detection(LOD)were 0. 32 μg/mL and 0. 08 μg/mL, respectively. The contents of osimertinib mesylate in samples were 100. 1%, 99. 5% and 99. 7%. Good chromatographic separation of osimertinib mesylate and its related substances, including synthetic impurities and degradation products, were obtained. The established HPLC method is specific, accurate, simple and durable, and could be used for the determination of osimertinib mesylate.