A dielectric barrier discharge (DBD) reactor as one of the advanced oxidation processes was applied to the degradation of diclofenac in aqueous solution. The various parameters that affect the degradation of diclofenac and the proposed evolutionary process were investigated. The results indicated that the inner concentrations of 10 mg/L diclofenac can be all removed within 10 min under conditions of 50 W and pH value of 6.15. The existence of Fe2+ in the liquid phase can promote the degradation of diclofenac. But it was rather ineffective in mineralization, because the intermediates containing the aromatic rings were recalcitrant to be degraded. Five intermediates were identiﬁed by liquid chromatography-mass spectrometry (LC-MS), the OH · radical and O3 were the major reactive species, and played an important role in the degradation of diclofenac. The toxicity of diclofenac degraded by DBD was assessed and the results indicated the efficiency of the DBD in the detoxification of the diclofenac solution.