The degradation of o-toluidine by Fenton process was examined by considering three main parameters including initial pH, ferrous ion concentration and hydrogen peroxide concentration. The effect of these parameters on o-toluidine and chemical oxygen demand (COD) removal efficiencies were studied. The results showed the optimum pH in this study is 3. Ferrous ion and hydrogen peroxide played an important role in Fenton process. The o-toluidine degradation efficiency increased with increased ferrous ion and hydrogen peroxide concentrations. The optimum condition for remove o-toluidine is 1 mM of ferrous ion and 5 mM of hydrogen peroxide. Results also revealed that the high concentration of hydrogen peroxide can enhance COD removal efficiency. Moreover, the highest o-toluidine removal for Fenton process was 100% in 40 min when applied 1 mM of ferrous ion and 50 mM of hydrogen peroxide at pH 3. The COD removal at this condition was 66% in 2 h with the rate constant about 0.51 mM-1 min-1. The degradation of o-toluidine followed the second-order behavior.