o-Toluidine is used primarily in the manufacture of dyestuffs, and also in the production of rubber, chemicals, and pesticides and as a curing agent for epoxy resin systems. It is considered to be toxic and carcinogenic. This study uses the Box–Behnken statistical experiment design to investigate the degradation of o-toluidine by the photo-Fenton process. The effects of ferrous ion dosage, hydrogen peroxide concentration and UVA irradiation were selected as factors in the Box–Behnken design experiment, while, o-toluidine and COD removal efficiency were considered as response functions. Results show that ferrous ion and hydrogen peroxide concentrations were the main parameters affecting the o-toluidine and COD removal, while the number of UVA lamps had a slight effect on the reaction. From the Box–Behnken statistical design prediction, the optimum condition for removing 100% of o-toluidine and 74% of COD was 1.2 mM of ferrous ion, 8 mM of hydrogen peroxide and UVA 85.7 W/m3 at pH 3.