Bactericidal magnetic nanoparticles were prepared by complexing iodine with poly(N-vinylpyrrolidone) (PVP) grown at the surface of silica coated magnetic nanoparticles (MNPs) via surface-initiated atom transfer radical polymerization (SI-ATRP). Approximately, 10 mg of iodine could be loaded onto one gram of the PVP-grafted MNPs to form bactericidal MNPs@PVP-I. At a concentration of 5 g L-1, MNPs@PVP-I could achieve 100% bactericidal rate for both Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus with a concentration of ∼1 × 1010 CFU mL-1 within 3 min. After being used for killing the bacteria in solution, the bactericidal rate of the MNPs@PVP-I decreased to <10% due to the consumption of iodine. The bactericidal rate could be tuned back to 100% when the used MNPs@PVP-I was recharged in a 15 g L-1 iodine solution for 12 h. The as-prepared bactericidal MNPs@PVP-I could be reused at least 4 times with 100% bactericidal rate by repeatedly recharging it with iodine.