Abstract: Magnetite nanoparticles were synthesized by chemical coprecipitation of ferrous chloride and ferric sulfate. The obtained magnetite nanoparticles were surface-modified with 3-methacryloxypropyltrimethoxy silane(3-MPS) to form terminal vinyl groups as grafting sites. The polystyrene(PS)-grafted magnetite nanoparticles were prepared by controlled/“living” free radical polymerization by using benzoyl peroxide (BPO) as an initiator in the presence of 4-hydroxyl-2,2,6,6-tetramethyl-1-piperidinyloxy(HTEMPO·). XRD measurement showed the spinel structure for the magnetite nanoparticles. GPC analysis suggested the relationship between number average molecular weight of PS and polymerization time to be with approximate linear. The PS-grafted magnetite nanoparticles are uniform with diameters in the range from 20 nm to 30 nm from TEM. The content of magnetite in magnetic PS nanoparticles was 62.6% from the analysis of TG. The results of magnetic performance by VSM displayed that the magnetic PS nanoparticles exhibited the characteristics of single domain system when saturated magnetization was equal to 31.7 emu·g-1.