Abstract: The urine sample was extracted with n-octanol with hexadecyl trimethyl ammonium bromide as auxiliary solvent. The extract was centrifugated, and then the upper organic phase was taken. HPLC was applied to the determination of 2 metabolites (4-chlorocatechol and p-chlorophenol) of monochlorobenzene in the upper organic phase. RD-C₁₈ chromatographic column was used as separation column, and a mixture of methanol and 0.15% (φ) phosphoric acid solution with volume ratio of 65∶35 was used as mobile phase. Detection at 280 nm was adopted in the determination. Linear relationships between values of peak area and mass concentration of 4-chlorocatechol and p-chlorophenol were kept in the same range within 600 μg·L^-1, with detection limits (3S/N) of 1.67, 2.23 μg·L^-1 and lower limits of determination (10S/N) of 5.55, 7.43 μg·L^-1 respectively. On the base of blank sample, test for recovery was made by standard addition method; values of recovery found were in the range of 89.2%-97.4%, with intra-day RSDs (n=6) and inter-day RSDs (n=18) of determined values in the ranges of 2.1%-4.9%, 2.5%-5.3%, respectively.