Poly (4-vinylpyridine) (P4VP) brushes had been prepared by the surface-initiated nitroxide-mediated radical polymerization of 4-VP on the surface of 3-methacryloxyproyltrimethoxysilane (3-MPS)-modified magnetite nanoparticles with an average diameter of 30 nm. The grafting polymerization was accomplished by nitroxide-mediated polymerization of 4-VP, using 4-hydroxyl-2,2,6,6-tetramethyl-1-piperidinyl-oxy (HTEMPO) free radical as capping agent and benzoyl peroxide (BPO) as initiator. X-ray photoelectron spectra (XPS) measurement demonstrated that the alkoxysilane initiator layer had formed on the magnetite surface. Gel permeation chromatograph analysis and XPS measurement suggested that the amount of grafted P4VP increases with increasing grafting time. The amount of P4VP grafted on the surface could be determined to be 0.09 chains/nm(2) by thermogravimetric analysis. The P4VP-grafted magnetite particles exhibited the characteristics of multidomain system, distinct from the single domain attributes of the pure magnetite particles. Atomic force-microscopy analysis revealed the diameter of the grafted P4VP magnetic latex particles is in the range of 120 nm to 150 nm. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 3582-3590, 2011