EPDM copolymer and method of preparing the same

An ethylene-propylene-diene monomer (EPDM) copolymer satisfying the following conditions i) to iii):

• i) a Mooney viscosity (ML1+4) of 60 or more at 150° C.,
• ii) an ethylene (C2) content of 45 to 70 wt %, and
• iii) a Mooney viscosity (ML1+4) of 45 or less as measured at 100° ° C. under the following EPDM composition conditions:
• the EPDM composition conditions of iii) being mixing 72 parts by weight of carbon black, 60 parts by weight of a paraffin oil, 50 parts by weight of CaCO3, 2 parts by weight of polyethylene glycol, 6 parts by weight of ZnO, and 2 parts by weight of a stearic acid, based on 100 parts by weight of the EPDM copolymer, at 50° C. at 60 rpm in a Banbury mixer.