The present invention relates to a method for the production of an ethylene/propylene/diene copolymer by means of solution polymerization, and more specifically one for a copolymer whose main components are ethylene, propylene and diene monomers using a catalyst of a transition metal coordination compound; the constituent components and characteristics of the copolymer being as follows. The catalyst composition provided by the present invention, which is used to produce a copolymer of which (a) the ethylene content is from 30 wt. % to 80 wt. %, (b) the propylene content is from 19.9% to 60 wt. % and (c) the diene content is from more than 0 wt. % to 15 wt. %, comprises a catalyst system using an activating agent of a compound of aluminum or boron, or aluminoxane together with a transition metal catalyst comprising at least one anionic ligand having an aryloxy group substituted with an aryl derivative in the ortho position and a cyclopentadiene derivative, and provides a highly active polymerization method with outstanding reactivity with respect to ethylene, propylene and diene compounds.