The Fenton oxidation process was applied as a pretreatment process to degrade non-biodegradable organic matters in pigment wastewater. It was necessary to continuously measure the fluctuating concentration of organics in the pigment wastewater and to determine the amount of Fenton’s reagent required to oxidize the organics. Batch and continuous flow tests were used to evaluate the relationship between the concentration of organics (COD_Cr) and the amount of Fenton’s reagent required to achieve a sufficient oxidation of the organics. On-line measurements of the oxidation-reduction potential (ORP) value in the batch and continuous flow tests showed that the maximum ORP values were highly related to the organic concentrations (expressed as COD_Cr) and the Fenton’s reagent dosage (expressed as H₂O₂ concentration). The empirical equation was [COD_Cr]=8808+0.494[H₂O₂]-14.6ORP. A control program of Fenton’s reagent dosage based on the empirical equation was applied to control of a pilot scale Fenton oxidation process using ORP measurement. The concentration of organics predicted with the control program well agreed with the observed concentration of organics in the pigment wastewater. The variation of the effluent organics concentration of the controlled Fenton oxidation process was significantly reduced compared to that of a process without the control system. These results suggested that the control system of Fenton’s reagent dosage using ORP measurement would be applicable to the Fenton oxidation process for efficient pretreatment of pigment wastewater.