Carboxyl-functionalized magnetic microspheres were prepared by in situ polymerization of styrene and methyacrylic acid at 85degreesC in the presence of nano-Fe3O4 in styrene, using lauroyl peroxide as an initiator. The effects of the surfactant, the weight ratio of magnetite to styrene, the amount of monomer and initiator on the polymerization reaction and microsphere properties were investigated in detail. Transmission electron microscopy showed the property of the surfactant on the surface of nano-Fe3O4 determined the magnetite content and the distribution of the magnetite in the microspheres. The optimum conditions of the reaction were optimized. The adsorption result of bovine-gammaIgG proves that the magnetic microspheres have a potential wide application in the biological field.