In the present paper, we report the successful preparation of high density, well-defined polymer brushes by RAFT polymerization on silica nanoparticles. Our strategy includes (i) "grafting from" approach for brush formation, (ii) facile click reaction to immobilize RAFT agent, (iii) synthesis of R-supported chain transfer agent (CTA) over Z-supported CTA, and (iv) use of more active trithicicarbonate RAFT agent over conventional dithioester type RAFT agent. With the advantage of the properties of high-density polymer brushes, these S diblock copolymer-modified silica nanoparticles open the route to the elaboration of a new family of nanocomposite colloids with potential applications in responsive or smart systems.