Fenton is the reaction using the OH· radicals generating by interaction between hydrogen peroxide and Fe
2+ which can oxidize the contaminants. Fe
2+ ions are oxidized to Fe
3+ ions by reaction with H
2O
2 and formed OH· radicals. UV-Fenton process includes the additional reaction that generates the OH· radicals by photodegradation of H
2O
2. In methylorange (MO) decolourization experiment with UV-Fenton, optimal Fe
2+: H
2O
2 ratio was obtained at 1 : 10. Based on the obtained condition (H
2O
2= 10mM, Fe
2+ = 1 mM) with/without UV-fenton experiment was carried out. Removal efficiency and sludge production were measured at 30 min. The case of w/o UV irradiation and only H
2O
2 was hardly treated and only Fe
2+ showed 65% removal owing to coagulation. When UV-Fenton process in optimal ratio (Fe
2+: H
2O
2 = 1 : 10), UV irradiation showed better removal efficiency than of w/o UV irradiation. Also, MO decolourization was a function of the hydrogen peroxide concentration (x
1), Fe
2+:H
2O
2 ratio (x
2), and numbers of UV lamp (x
3) from the application of the response surface methodology. Statistical results showed the order of significance of the independent variables to be hydrogen peroxide concentration > numbers of UV l amp > Fe
2+: H
2O
2 ratio.
Keywords: Fenton;UV;Response surface mothodology;