Abstract: A method for determination of 3-nitrophenol and 4-nitrophenol in surface water by high performance liquid chromatography combined with fluorescence detector after online pre-column reduction was proposed. 500 mL of water sample was placed into a separating funnel, and 30 g of sodium chloride was added. The pH of the mixture was adjusted to less than 2 with 10% (mass fraction) hydrochloric acid solution. 40 mL of a mixture of dichloromethane and ethyl acetate at a volume ratio of 1∶1 was added, the mixture was shaken for 5 min and then settled, and the organic phase was collected, which was extracted repeatly 3 times. The organic phases were combined, and dehydrated with anhydrous sodium sulfate. The extract was collected, and rotary evaporated to near dryness at 40 ℃ under 79 kPa. The residue was re-dissolved and made its volume up to 1.0 mL with methanol. Hypersil GOLD chromatography column was connected with a zinc powder reduction column of packing particle size of 70 µm, and methanol solution containing 0.39 g·L⁻¹ acetic acid, 0.53 g·L⁻¹ zinc acetate,and 0.083 g·L⁻¹ copper sulfate was used as mobile phase for separation. The reduction products of 3-nitrophenol and 4-nitrophenol were determined by fluorescence detector. As shown by the results, linear relationships between the peak areas of the corresponding reduction products and mass concentrations of 3-nitrophenol and 4-nitrophenol were found within 10-100 µg·L⁻¹, with detection limits (3S/N) of 3.0, 1.5 µg·L⁻¹, respectively. Test for recovery was made by the standard addition method, giving results in the range of 80.0%-92.6%, with RSDs (n=6) of the determined values not more than 11%.