Background & Objective: Reactive dyestuff causes difficulties including toxicity, carcinogenesis, and mutagenesis in the environment. Traditional physical and chemical treatment methods such as coagulation, flocculation, and adsorption can’t destroy these compounds and cause secondary environmental pollution. Advanced oxidation processes like Fenton oxidation are one of the most efficient processes for dyestuff pollutants removal from aqueous solution. In this study, the ability of Remazol Black- B (RB-B) removal from aqueous medium by Fenton oxidation process was investigated. Methods: This experimental-intervention study was carried out in laboratory scale. In order to determine the effect of Fenton oxidation on decolorization of RB-B, optimum levels of variables such as pH, FeSO4 and H2O2 and reaction time was measured in a 1 L batch reactor. Results: The results of the experiments showed that for 100 mg/L of dyestuff the respective optimum concentrations of FeSO4 and H2O2were10 and50 mg/L. For 200 and 500 mg/L of dyestuff they were50 and300 mg/L, respectively. Under the optimal condition, pH=3 and 15 min of reaction time, up to 95% decolorization efficiency was achieved for all three concentrations of dyestuff. Conclusion: Based on the results of this study, the Fenton process is an effective method for the treatment of different concentrations of RB-B dyestuff in wastewater. The selection of proper concentration of Fenton is important to reduce treatment costs, environmental problems, and sludge produced.