The chemical oxygen demand (COD) removal efficiency for treatment of an oil-water emulsion from the machinery industry using Fenton's reagent was investigated. The optimal [H 2 O 2 ]/[Fe 2+ ] molar ratio for COD removal was 3. An orthogonal test was designed based on the optimal molar [H 2 O 2 ]/[Fe 2+ ] ratio to evaluate the significance of four parameters relevant to the treatment process, namely, H 2 O 2 dosage, initial pH, oxidation time and coagulation pH. The influence of the following parameters on COD removal efficiency decreases as follows: H 2 O 2 dosage > oxidation time > coagulation pH > initial pH. The COD removal efficiency was investigated based on the most important single-factor parameter which was H 2 O 2 dosage, as discovered in the orthogonal test. A well-fitted empirical correlation was obtained in the single-factor analysis and up to 98% COD removal was attained using 50 mM H 2 O 2 . With the increase of H 2 O 2 dosage up to 50 mM, the COD removal efficiency increased rapidly due to a higher generation of hydroxyl radicals. However, this effect becomes less significant as the dosage of H 2 O 2 increases. This effect might be attributed to the consumption of hydroxyl radicals with excess H 2 O 2 . The coagulation function of Fenton's reagent was confirmed by scanning electron microscope (SEM). Using the doses and conditions identified in this study, the treated oil-water emulsion can be discharged according to Chinese Standard JS-7740-95.