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 2 O 2 and formed OH· radicals. UV-Fenton process includes the additional reaction that generates the OH· radicals by photodegradation of H 2 O 2 . In methylorange (MO) decolourization experiment with UV-Fenton, optimal Fe 2+ : H 2 O 2 ratio was obtained at 1 : 10. Based on the obtained condition (H 2 O 2 = 10 mM, 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 2 O 2 was hardly treated and only Fe 2+ showed 65% removal owing to coagulation. When UV-Fenton process in optimal ratio (Fe 2+ : H 2 O 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 2 O 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 2 O 2 ratio.