Magnetic nano-scaled particles Fe 3 O 4 were studied for the activation of peroxymonosulfate (PMS) to generate active radicals for degradation of acetaminophen (APAP) in water. The Fe 3 O 4 MNPs were found to effectively catalyze PMS for removal of APAP, and the reactions well followed a pseudo-first-order kinetics pattern (R 2 >0.95). Within 120min, approximately 75% of 10ppm APAP was accomplished by 0.2mM PMS in the presence of 0.8g/L Fe 3 O 4 MNPs with little Fe 3+ leaching (<4μg/L). Higher Fe 3 O 4 MNP dose, lower initial APAP concentration, neutral pH, and higher reaction temperature favored the APAP degradation. The production of sulfate radicals and hydroxyl radicals was validated through two ways: (1) indirectly from the scavenging tests with scavenging agents, tert-butyl alcohol (TBA) and ethanol (EtOH); (2) directly from the electron paramagnetic resonance (ESR) tests with 0.1M 5,5-dimethyl-1-pyrrolidine N-oxide (DMPO). Plausible mechanisms on the radical generation from Fe 3 O 4 MNP activation of PMS are proposed based on the results of radical identification tests and XPS analysis. It appeared that Fe 2+ Fe 3+ on the catalyst surface was responsible for the radical generation. The results demonstrated that Fe 3 O 4 MNPs activated PMS is a promising technology for water pollution caused by contaminants such as pharmaceuticals.