Based on "ligand-oriented catalyst design", group 4 transition metal complexes having two phenoxy-imine ligands, Fl Catalysts, were synthesized and investigated as olefin polymerization catalysts using ethylene at atmospheric pressure. As a result, these complexes were found to exhibit very high activity using MAO as a cocatalyst. Among them, zirconium complexes displayed the highest activity with moderate (Mv: 0.8 x 10(4)) to very high (Mv: 71.6 x 10(4)) molecular weight ranges. The maximum activity exceeded 4 t-PE/(mmol-cat(superscript:)h) at 25 degreesC even at atmospheric pressure, the activity being two orders of magnitude larger than that exhibited by Cp2ZrCl2. Alternatively, by using borate/(Bu3Al)-Bu-i as a cocatalyst, a zirconium Fl Catalyst produced exceptionally high molecular weight polyethylene (Mv: 505 x 10(4)) displaying considerable activity. As far as we know, this is one of the highest molecular weight values obtained from homogeneous polymerization catalysts. These results indicate that Fl Catalysts possess very high potential as olefin polymerization catalysts.