Well-defined poly(methyl methacrylate)-silica nanocomposites were produced by reversible addition –fragmentation chain transfer (RAFT) polymerization by grafting “through” and “from” surface-modified silica nanoparticles. The surface of silica nanoparticle is modified by covalently attaching RAFT agent or methacryl group to the surface, and polymerization of methylmethacrylate (MMA) produces the PMMA-SiO2 nanocomposites. The characterization of these well-defined nanocomposites is done by FT-IR, gel permeation chromatography (G.P.C), thermogravimetric analysis (TGA), and Differential scanning calorimeter (D.S.C).