The residues of sulfonamides and their metabolites can not only give rise to the generation of resistant bacterial/gene, but also have negative impacts on both human and ecosystem. In this work, the oxidation of sulfadiazine (SDZ) using permanganate as an oxidizer was comprehensively investigated, including the degradation kinetics, pathways and byproducts toxicity evaluation. In general, the oxidation of SDZ follows the pseudo first-order kinetics with the rate constant of 0.00168-0.498 min(-1) . The activation energy was calculated to be 58.7 kJ mol(-1) based on Arrhenius equation. Higher SDZ removal rate can be achieved under the lower pH condition, and the removal rate was decreased accompanying with increasing pH from 3.76 to 8.80. The presence of humic acid at low concentration can accelerate SDZ oxidation rate. Six oxidized byproducts were identified and the plausible oxidation pathways were also proposed. The quantum chemistry calculations indicate that S (2), N (14) and C (16) were more likely to be attacked, which were consistent with the proposed pathways. Furthermore, the toxicity evaluation of the oxidation byproducts suggests that the toxicity of three byproducts (i.e. 2-aminopyrimidine, 4-nitro-N-(pyrimidin-2-yl) benzene sulfonamide and 4-(2-iminopyrimidin-1(2H)-yl) aniline) was higher than parent compound. Despite of complete removal by permanganate, the byproducts that generated by SDZ were even more toxic than parent compound. Therefore, more attentions should be paid to application of permanganate to remove sulfadiazine and further risk assessments should be made to transformed byproducts.