Magnetic block copolymers (PNIFe‐b‐PNC) with a well‐defined and controllable structure were synthesized via ring opening metathesis polymerization (ROMP) and post modification with ferric trichloride hexahydrate (FeCl· 6HO). The magnetic properties, surface morphology and antimicrobial activities for Gram‐positive bacteria and Gram‐negative bacteria of magnetic copolymers were studied. All of the magnetic copolymers were characterized of paramagnetic property. The susceptibilities were affected by the content of imidazolium and the degree of polymerization of copolymers. The maximum magnetic susceptibility was 20.96 × 10emu/g (PNIFe‐b‐PNC). The magnetic copolymers showed high thermal stability and start to decompose at 35065°C. The magnetic copolymers showed higher and broader‐spectrum antimicrobial activity, which leads to the well‐defined magnetic polymers materials having the potential applications in wide fields such as antimicrobial films.