Polystyrene (PS)-grafted silica nanoparticles were prepared by nitroxide-mediated polymerization of styrene using N-tert-butyl-N-[1-diethylphosphono(2,2-dimethylpropyl)] nitroxide (DEPN) as mediator. Two routes were investigated to graft the alkoxyamine initiator onto silica. In the first route, (acryloxypropyl)trimethoxysilane (APTMS) was covalently attached to silica and the alkoxylamine was formed in situ by spin trapping the acryloxy radicals produced by reaction of azobisisobutyronitrile (AIBN) with the grafted APTMS molecules using DEPN as radical trap. In the second route, the surface alkoxyamine initiator was produced in a one-step process by reacting simultaneously DEPN, AIBN, and APTMS in the presence of silica. Next, polystyrene chains with controlled molecular weights and narrow polydispersities were grown from the alkoxyamine-functionalized nanoparticle surface. The amount of polystyrene grafted to the surface was determined by thermogravimetric analysis, and was found to increase with increasing grafting density of the alkoxyamine initiator. The resulting PS-grafted silica particles exhibited better colloidal stability and enhanced dispersability in toluene, a good solvent for polystyrene.