Well-defined polymer brushes grafted onto silica nanoparticles were prepared by reversible addition-fragmentation chain transfer polymerization (RAFT). A versatile RAFT agent, 4-cyanopentanoic acid dithiobenzoate (CPDB), was attached to amino-functionalized colloidal silica nanoparticles by direct condensation of the mercaptothiazoline-activated CPDB with the surface amino groups. RAFT polymerizations were then conducted from the particle surface to graft polymer brushes to the particles. The kinetics of methyl methacrylate and styrene surface RAFT polymerizations were investigated and compared with model polymerizations mediated by free CPDB. The MMA surface graft polymerization was more controlled than the solution polymerization mediated by free RAFT agent, indicated by the faster polymerization rate and narrower polydispersity. High-performance liquid chromatography techniques were used to quantitatively estimate the amount of ungrafted free polymer which was minimal compared with the grafted polymer.