Ethylene-propylene-diene monomer (EPDM) rubber composites were prepared using two grades of carbon black (CB) with different particle sizes to investigate the influence of filler characteristics on composite properties. The composites were fabricated by melt mixing and hot pressing, and their dispersion behavior, mechanical properties, and thermal stability were evaluated. Both CB grades were uniformly dispersed in the EPDM matrix, resulting in improved mechanical performance compared with EPDM. Notably, the composite containing 45 phr of larger-particle-size CB exhibited higher tensile strength and modulus, attributed to effective filler network formation and enhanced stress transfer at the filler-rubber interface. Thermogravimetric analysis showed comparable thermal stability for all composites. These results indicate that carbon black effectively reinforces EPDM, and that optimizing CB particle size and loading is important for developing impact-resistant rubber composites.