Well-defined core-shell structured poly(methyl methacrylate) grafted silica nanocomposites (PMMA-g-SiO2) were synthesized via thiol-lactam initiated radical polymerization by employing a grafting from protocol. Spherical SiO2 nanoparticles were reacted with 3-mercaptopropyl-trimethoxysilane in order to prepare thiol functionalized SiO2 nanoparticles (SiO2-SH). Subsequently, the surface initiated polymerization was accomplished by using two component initiating system comprising SiO2-SH and butyrolactam. The anchoring of PMMA onto the surface of SiO2 nanoparticles was ascertained by FTIR and XPS analyses. A uniform and spherical core-shell structure of PMMA-g-SiO2 was revealed from TEM and SEM images. The GPC analysis demonstrated that the molecular weight of PMMA was a function of both monomer conversion and polymerization time. DLS also confirmed that the hydrodynamic diameter of the nanocomposites was directly related to the polymerization time. An improved thermal property of the polymer matrix with incorporating SiO2 nanoparticles was revealed by TGA study.