Precursor particles, monodisperse magnetic microspheres with diameter of 0.52 µm, were produced by deposition of Fe3O4 nanoparticles within crosslinked poly(acrylic acid) microspheres prepared by distillation–precipitation copolymerization. Subsequently, magnetic microspheres with atom transfer radical polymerization (ATRP) initiators were prepared by coupling the amino group of ethanolamine with the carboxyl group on the precursor particles followed by esterification of the hydroxyl group on the microspheres with 2-bromoisobutyryl bromide. tert-Butyl acrylate, glycidyl methacrylate and styrene were initiated by the microsphere initiators to form polymer brushes on the particles. The polymer brushes formed in both the surface layer and the interior, while the density and chain length of the polymer brushes showed a positive gradient distribution from the interior to the surface layer. Diblock copolymer brushes were also prepared using homopolymer-grafted magnetic microspheres as initiators. Poly(acrylic acid)-grafted magnetic microspheres were prepared by treating poly(tert-butyl acrylate)-grafted magnetic microspheres with trifluoroacetic acid. A mixture of glycidyl methacrylate and glycerol monomethacrylate was initiated by the microsphere initiators. The resulting magnetic microspheres possess poly(glycidyl methacrylate) units, which act as functional groups, and poly(glycerol monomethacrylate) units, which are hydrophilic and make the microspheres dispersible in aqueous solution. Bovine serum albumin as a model protein was immobilized onto the random copolymer-grafted magnetic microspheres.