Disclosed is a homogeneous catalyst system for producing an ethylene homopolymer or an ethylene copolymer with α-olefin. Specifically, this invention pertains to a transition metal catalyst which has stability under high temperature solution polymerization at 120˜250° C., in which a cyclopentadiene derivative and an electron donating substituent, both of which are bonded to a Group IV transition metal acting as a central metal, are crosslinked through a silyl derivative substituted with a cyclohexyl, to a catalyst system including such a transition metal catalyst and an aluminoxane cocatalyst or a boron compound cocatalyst, and to a method of producing an ethylene homopolymer or an ethylene copolymer with α-olefin, having high molecular weight, using the catalyst system under conditions of high-temperature solution polymerization. The catalyst according to this invention has excellent thermal stability and compatibility with a paraffin hydrocarbon solvent and thus is effective in the production of an ethylene homopolymer or an ethylene copolymer with α-olefin having various properties in commercial polymerization processes.