The use of nanoscale zero-valent iron (nZVI) for the purification of contaminated water and soil was investigated for its high potential to remove subsurface contaminants. In this study, the mechanism of diclofenac (DCF) degradation by nZVI and the combined process of nZVI and hydrogen peroxide (H₂O₂) are discussed. The results demonstrated that nZVI with acidic conditions had a certain degradation efficiency on DCF, with a removal rate about 30% in a 2-hour reaction at pH 5. The combined process of nZVI and H₂O₂ could obviously remove DCF, with a removal rate greater than 90% in a 2-hour reaction at pH 5. In nZVI system, under acidic and aerobic conditions, Fe²⁺ and H₂O₂ can be generated to form a Fenton-like system. Meanwhile, the release of electrons from nZVI could promote the reductive dechlorination of DCF. In nZVI/H₂O₂ system, the high removal rate of TOC was also showed the DCF mineralization by the Fenton-like oxidation.