Nitroxide-mediated stable free radical polymerization of styrene was performed from 13 nm diameter silica nanoparticles in two steps. First, an alkoxyamine, based on N-tert-butyl-N-(1-diethylphosphono-2,2-dimethylpropyl) nitroxide (DEPN) carrying a terminal triethoxysilyl functional group was synthesized and covalently attached onto silica. Polystyrene chains with controlled molecular weights and narrow polydispersities were then grown from the alkoxyamine-functionalized nanoparticles surface. The grafting of both the initiator and the polystyrene was qualitatively evidenced by FTIR spectroscopy and solid-state NMR and quantified by thermogravimetric and elemental analysis. More than 40% of the surface bound alkoxyamine initiator participated in the growth reaction. Polymer graft densities of around 110 chains per particle were obtained by this technique.