In the present investigation a new method is described for modification of dental resins with reactive polymeric nanoparticles (RPNPs) as organofillers. Nanoparticles (NPs) as model compounds indicate clearly that the RPNPs significantly influence the mechanical and shrinkage properties of the matrix and composite. Nanoparticles were prepared by free radical non-linear copolymerization of a mixture of mono (styrene, ST) and bifunctional (ethylene glycol dimethacrylate, EGDM) comonomers in homogeneous solution. The EGDM monomer is a commonly used component of several dental filling composites. The size, reactivity and distribution of nanoparticles are influenced by the mole ratio of the monomers in the feed. Size of nanoparticles was determined by Scanning Electron Microscopy (SEM). Reactivity of nanoparticles was measured by Nuclear Magnetic Resonance (NMR).