Volumetric shrinkage during polymerization has always caused problems since polymer based composites are used as dental filling materials. This value has been reduced to 2-4 volume % with the improvement of dental filling materials and of suitable filling technique. The purpose of our study was to synthesize reactive polymeric nanoparticles which can be applied to modify the polymerization shrinkage of dental resin. It means that certain part of a commercial dental resin would be substituted with reactive polymeric nanoparticles as organic filler. According to our hypothesis the volume contraction would significantly be reduced by applying the modified resin with organic filler. Nanoparticles were prepared by free radical copolymerization from mono-, di- and trifunctional monomers in emulsion and organic solution. In emulsion their size distribution is narrow, but it is quite variable in solutions. The size, size distribution and reactivity of nanoparticles were examined, and these properties can be influenced by the mol ratio and concentration of the monomers. It was shown that the nanoparticles could be swollen by the matrix material and these build a unique structure that can be photo-polymerized. During polymerization the shrinkage of the copolymer can be considered zero, consequently the total contraction originates from the shrinkage of the matrix. In the modified resin the reactive groups of monomers and the pendant double bounds of nanoparticles were connected by covalent bond.