The most widely used reinforcing nanomaterial for the production of tires and other rubber products is carbon black. Their production processes as well as the aggregate dimension determine their unique properties which can be used like a UV stabilizer, conductive or insulating agent in polymer composites. The degree of their dispersion, surface activity, structure, type, and their amount in the matrix also play an important role in their properties. The most commonly used property is their stiffening effect in the polymer matrix, which is influenced by the interaction between polymer molecules and carbon black particles (Spahr and Rothon 2016; Song et al. 2017). This study deals with the preparation of polymer blends with different types of industrially used carbon blacks of the type N121, N339, N550, and N660. The influence of the type and amount of carbon black (45; 50; 55; 60 phr) on the properties of polymer blends based on natural rubber was investigated. The prepared blends were studied for hardness, tensile strength, and elongation at break before and after thermo-oxidation aging. The amount of filler in the polymer blends mainly affected the hardness, which increased with the increasing filler amount, but hardness values decreased with increasing filler size in blends. The amount and type of filler also affected tensile strength and elongation at break of polymer blends before and after thermo-oxidation aging.