Analyzed the community assessment fractal structure and properties of dispersion-filled polymer composites (with fillers having a size of micron-scale) and polymer nanocomposites. It is shown that in the framework of the fractal approach, the main difference of the mechanical properties of these materials is due to the type of space in which their structure is formed. Because of this, hardening of polymer composites is the result of modifying the structure of the matrix polymer and reinforcement nanocomposites – a consequence of the formation of interfacial layers.