One of the advanced oxidation methods is the Fenton’s reaction which uses a mixture of iron(II) and hydrogen peroxide to generate hydroxyl radicals. The Fenton process efficiency is influenced by a number of factors including the pH, the Fe²⁺:H₂O₂ relation or the Fe²⁺ dose. The aim of the study was to optimize the Fenton method in the application for landfill leachate treatment, with the landfill in Janczyce (Świętokrzyskie Voivodeship) as an example. The sequence of determining the most advantageous conditions for the use of Fenton’s reagent was as follows: A – the pH effect; B – the oxidation time effect; C – the Fe²⁺:H₂O₂ relation effect; D – the Fe²⁺ dose effect. The optimum catalyst/oxidant ratio of 0.125 was determined. The optimum process time of 90 minutes was determined. It was demonstrated that the Fenton process can be effectively carried out already at the pH of 4. A reduction by 75% for the TOC and by 89% for the COD, respectively, was obtained.