In this study, the compatibility of nano-silica in a PDMS matrix was improved by using poly(dimethyl siloxane) (PDMS) grafted by poly(hydroxypropyl acrylate)(PHPA) synthesized via in-situ dispersion polymerization that led to a good dispersion and distribution of the nano-silica in a typical PDMS matrix compared to unmodified PDMS samples. During the polymerization PDMS macromonomer was used for the stabilization of PHPA particles in cyclohexane as nonpolar medium and nano-silica. The surface properties of samples were characterized by Attenuated Total Reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray Photoelectron Spectroscopy (XPS) and Scanning Electron Microscopy (SEM). Dynamic Mechanical Thermoanalysis (DMTA) and tensile test were used to study the physical and mechanical properties of the PDMS/silica nanocomposites. Grafting of PHPA on the surface was confirmed using ATR-FTIR and XPS. The results also showed that the mechanical properties of the modified samples increased by the addition of PHPA. Keyword: In-Situ Polymerization, Nano-silica, PDMS, Grafting Nanocomposite, Mechanical Properties.