Abstract: The water sample was passed through 0.45 μm membrane, and 1.0 mL of the filtrate was taken, and mixed evenly with 10 μL of formaldehyde. The mixture was settled in the dark for 5 min for derivatization, and the resulting solution was used for high performance liquid chromatography analysis. The separation was performed on PAHs column (250 mm × 4.6 mm, 5 μm) using the mixed solution consisting of potassium dihydrogen phosphate buffer solution (pH 7.0) and acetonitrile at a volume ratio of 85∶15 as the mobile phase for isocratic elution. Quantification was carried out by external standard method at detection wavelength of 240 nm. It was shown that linear relationship between the mass concentration of 1,1-dimethylhydrazine and peak area of its derivative was kept in the range of 20-2 500 μg · L⁻¹, with detection limit (3.143s) of 2.0 μg · L⁻¹. The spiked recovery tests were conducted on surface water and wastewater matrices at mass concentrations of 20, 200, 2,000 μg · L⁻¹, giving recoveries in the range of 82.6%-105% and 91.7%-104%, and RSDs (n=6) of the determined values in the ranges of 0.62%-2.1% and 0.58%-2.9%. The proposed method was applied to the analysis of 7 actual samples, and the determined results showed no significant difference compared with those obtained by the national standard method GB/T 14376—93.