Abstract: After reaction of the silicane coupling agent with the hydroxyl on the surface of graphite nanoplatelets(GNPs), polymethyl methacrylate (PMMA) was chemically grafted to the GNPs by using atom transfer radical polymerization(ATRP). The microstructure change of raw GNPs and grafted GNPs was investigated by SEM, TEM, FTIR and XRD. The grafted GNPs were added into the epoxy resin, and the mechanical properties and dimensional stability of GNPs-PMMA/epoxy composites were studied. The PMMA grafted GNPs show better reinforcing effect on the mechanical properties of epoxy resin in contrast with raw GNPs. The tensile strength and modulus of epoxy resin increase by 17.4% and 75% respectively after being added 0.5 wt% GNPs-PMMA. At the same time, the bending strength and modulus of epoxy resin exhibit enhancement of 6% and 12%, respectively. And the linear coefficient of thermal expansion (CTE) of epoxy resin declines by 25% below the glass transition temperature.