Abstract: The sample powder (10 mg) was ultrasonically extracted with 5 mL of methanol. After centrifugation, the supernatant was separated on a Waters ACQUTIY UPLC BEH C₁₈ column (2.1 mm×50 mm, 1.7 μm), with methanol containing 0.1% (φ) formic acid and water with volume faction of 87:13 as mobile phase for isocratic elution at a flow rate of 0.2 mL·min^-1. Δ9-Tetrahydrocannabinol (Δ9-THC) and Δ9-tetrahydrocannabinolic acid A (Δ9-THCA-A) were qualitatively and quantitatively analyzed by photodiode array detector at the wavelength of 220 nm and confirmed by mass spectrometer QDa. Linear relationships were found between peak areas and mass concentrations of Δ9-THC and Δ9-THCA-A in the range of 0.50-20.0 mg·L^-1, 2.0-40.0 mg·L^-1, respectively, with detection limits (3S/N) of 0.10, 1.0 mg·L^-1, respectively. Values of recovery obtained by standard addition method were in the range of 81.5%-97.9%, and RSDs (n=6) were in the range of 0.77%-4.1%. The proposed method was applied to analysis of 20 cannabis plant samples, the total Δ9-THC contents calculated with the determination results of Δ9-THC and Δ9-THCA-A were slightly high than the determination results obtained by the standard method.