This invention relates to a cycloolefin copolymer, which is prepared via ring-opening metathesis polymerization using, as monomers, dicyclopentadiene obtained by chemically bonding two cyclopentadiene molecules using a Diels-Alder reaction and tricyclopentadiene obtained by chemically bonding three cyclopentadiene molecules using a Diels-Alder reaction, in which the cyclopentadiene is a C5-fraction of naphtha cracking, and to a method of preparing the same. This copolymer is a non-crystalline transparent resin and is useful for a variety of end uses.