Provided are an ethylene-propylene-diene monomer (EPDM) copolymer and a method of preparing the same. An EPDM copolymer which has more improved miscibility with inorganic fillers such as carbon black, may further facilitate dispersibility in mixing to further decrease the viscosity of a compound composition, and may provide a compound composition having excellent processability and mechanical properties. A method of preparing the same is also provided.