Abstract: Objective To develop a sensitive and reproducible HPLC-MS/MS method for analyzing dimemorfan in human plasma and urine. Methods Dimemorfan was extracted from plasma and urine by redistilled ether, with lidocaine serving as the internal standard (IS). The analysis was performed on a column of ultimate C₁₈ (50 mm×4.6 mm, 5 μm) with the mobile phase consisting of methyl alcohol-water-formic acid=75∶25∶0.05 at a flow rate of 0.2 mL/min. Dimemorfan was detected by API 3000 mass spectrometer, with multiple reaction monitoring after protonated with ESI in positive electron ionization mode. The ion pairs being detected were (m/z) 256.4→155.3 (dimemorfan) and 235.4→86.1 (lidocaine), respectively. Results The regression equation for dimemorfan showed excellent linearity ( r=0.995 7) from 0.025 to 5.0 ng/mL of plasma with detecting limitation of 0.025 ng/mL and perfect linearity ( r=0.998 3) from 0.1 to 20.0 ng/mL of urine with detecting limitation of 0.1 ng/mL. The method recoveries of dimemorfan in plasma and urine were ranging from 103.38% to 106.88% and 90.05% to 101.40%, respectively. The maximum intra-day and inter-day relative standard deviations (RSD) of concentration of dimemorfan were 5.92% and 5.70% (for plasma), 10.35% and 8.80% (for urine), respectively. Conclusion This new method was validated to be accurate and sensitive to determinate the concentration of dimemorfan in plasma and urine samples, and can be applied for pharmacokinetic studies of dimemorfan.