Chemical modification of magnetic nanoparticles(MNPs) with functional polymers has recently gained a great deal of attention because of the potential application of MNPs to in vivo and in vitro biotechnology. The potential use of MNPs as capturing agents and sensitive biosensors has been intensively investigated because MNPs exhibit good separation-capability and binding-specificity for biomolecules after suitable surface functionalization processes. In this work, we demonstrate an efficient method for the surface modification of MNPs, by combining surface-initiated polymerization and the subsequent conjugation of the biologically active molecules. The polymeric shells of non-biofouling poly(poly(ethylene glycol) methacrylate)(pPEGMA) were introduced onto the surface of MNPs by surface-initiated, atom transfer radical polymerization(SI-ATRP). With biotin as a model of biologically active compounds, the polymeric shells underwent successful post-functionalization via activation of the polymeric shells and bioconjugation of biotin. The resulting MNP hybrids showed a biospecific binding property for streptavidin and could be separated by magnet capture.