It is very difficult to remove high concentration, non-biodegradable organic substances from the leachate of a stabilized landfill, by biological treatment. The purpose of this paper is to find proper conditions for their removal by using the Fenton oxidation reaction. The ratio of BOD5/COD in the raw water was between 0.11 and 0.17. The optimum condition of the Fenton oxidation process using various coagulants, Fe2+, and Fe3+ was decided by the treatment efficiency of organic substances. When the proper conditions for using coagulant were met, 1,500 mg/l of FeCl3 at PH 5 was added. When the proper conditions for Fenton oxidation using Fe2+ as a catalyst were met, we added 1, 500 mg/l of H2O2 and 2, 000 mg/l of Fe2+ at PH 3, and then, using Fe3+ we added 1,200 mg/l of H2O2 and 1,500 mg/l of Fe3+. The COD removal rate of leachate was between 45.1% and 50% when the Fenton oxidation reaction was applied by itself, but the proper conditions were reached using, 500 mg/l of H2O2 and 600 mg/l 1 of Fe2+ at PH 3, when the coagulation process and Fenton oxidation process using Fe2+ as a catalyst were put together. At this time the ratio of BOD5/COD was improved to 0.45 and this process is observed to be applicable as a biological pre-treatment.