Abstract: A HPLC external standard method for quantitative analysis of imidazole and the 2 impurities of 2-methylimidazole and 4-methylimidazole in reaction solutions in the production process of imidazole was established. In the HPLC analysis, Supersil-ODS-B column was selected as stationary phase, and mixture of iacetonitrile and ionic reagent solution (pH 3.5) containing 16 mmol·L^-1 of sodium dodecyl sulfate and 17 mmol·L^-1 of potassium dihydrogen phosphate at a volume ratio of 40:60 was used as mobile phase in isocratict elution. Diode array detector was used for detection, and the detection wavelength was 210 nm. As shown by the results, linearity ranges of standard curves of imidazole, 2-methylimidazole and 4-methylimidazole were same in the range of 10-100 mg·L^-1, and detection limits (3S/N) found was 0.02, 0.02, 0.03 mg·L^-1 respectively. The average recoveries found at the addition levels of 20 mg·L^-1 and 100 mg·L^-1 were 100% and 99.2% respectively, and RSDs (n=8) of the determined values was 0.27%. The imidazole in 9 samples of different reaction stages of the same batch of reaction solution was determined by the present method, giving determined values ranged from 10.11% to 10.71%. The impurity of 2-methylimidazole was detected, but its content was less than the lower limit of determination. 4-methylimidazole was not detected. This method was compared with the gas chromatography method, giving a consistent result. The introduced method was accurate, sensitive and reproducible, which was applied to rapidly determine imidazole, 2-methylimidazole and 4-methylimidazole in the reaction solution in the production process of imidazole.