Magnetic nanoparticles were studied to generate novel nanocomposites which maintain both magnetic properties of nanoparticles and self-assembly of amorphous block copolymer matrix. With this goal, iron oxide magnetic nanoparticles were modified with polystyrene (PS) brushes by atomic transfer radical polymerization (ATRP) to improve both the dispersion and the affinity of nanoparticles with one of the blocks of polystyrene-b-polybutadiene-b-polystyrene block copolymer. This way of preparation of nanocomposites opens new strategy to the generation of magnetic nanocomposites. The samples were characterized with the use of differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), rheological measurements, and atomic force microscopy (AFM).