Copolymerizations of ethylene and ol-olefin with various zirconocene compounds at a high temperature were carried out to study the relationship between the ligand structure of zirconocene compounds and the copolymerization behavior. All of the indenyl-based zirconocene compounds in combination with dimethylanilinium tetrakis(pentafluorophenyl)borate/triisobutylaluminium produced only low molecular weight copolymers at a high temperature; regardless of the substituents and bridged structures of the zirconocene compounds. However, zirconocene compounds with a fluorenyl ligand gave rise to a significant increase in the activity and molecular weight of the copolymers by the selection of a diphenylmethylene bridge structure even at a high temperature. Ethylene/1-hexene copolymers obtained with the fluorenyl-based catalysts contained inner double bonds accompanied by the generation of hydrogen, presumably because of a C-H bond activation mechanism. The contents of the inner double bonds were significantly influenced by the polymerization conditions, including the 1-hexene feed content, polymerization temperature, and ethylene pressure.