Antibiotics are known today as emerging contaminants due to potentially adverse effects on aquatic ecosystems and the health of humans and animals, even at very low concentrations. The present study was conducted to evaluate the efficiency of the UV/S2O8 process and affecting factors (pH, initial metronidazole (MNZ) concentration, initial persulfate concentration and reaction time) in removing antibiotic MNZ. The results obtained from the experiments showed that the UV/S2O8 process efficiency is higher in acidic pH values due to production of further radical SO4− and increases with extended contact time, but the efficiency of the process is reduced by increasing the concentration of MNZ. In assessing the effect of initial persulfate concentration on the process efficiency, MNZ removal efficiency was also increased by 99.5% after contact time of 35 min with increasing the initial persulfate concentration up to 1 g/L. However, the process efficiency was decreased at higher concentrations (2 mg/L) due to reaction of sulfate radicals with each other or with persulfate and its saturation. The kinetic data fitted the pseudo-first-order kinetic model (R2 > 99%). The findings of this study clearly demonstrated the high potential of the UV/S2O8 process in the degradation of MNZ.