Disclosed is an arylphenoxy catalyst system for producing an ethylene homopolymer or copolymers of ethylene and .alpha.-olefins, and a method of producing an ethylene homopolymer or copolymers of ethylene and .alpha.-olefins having a high molecular weight under a high temperature solution polymerization condition using the same. The catalyst system includes a group 4 arylphenoxy-based transition metal catalyst and an aluminoxane cocatalyst or a boron compound cocatalyst. In the transition metal catalyst, a cyclopentadienyl derivative and arylphenoxide as fixed ligands are located around the group 4 transition metal, arylphenoxide is substituted with at least one aryl derivative and is located at the ortho position thereof, and the ligands are not crosslinked to each other. The catalyst includes non-toxic raw materials, synthesis of the catalyst is economical, and thermal stability of the catalyst is excellent. It is useful for producing an ethylene homopolymer or copolymers of ethylene and .alpha.-olefins having various physical properties in commercial polymerization processes.