Ethylene (Et) polymerization and Et/alpha-olefin copolymerization were carried out with various hafnocenes activated with dimethylanilinium tetrakis(pentafluorophenyl)borate (Me2PhNH . B(C6F5)(4)/triisobutylaluminum (i-Bu3Al) to study the relationship between ligand structures and catalyst performance at high temperature. Dimethylsilylene(bisindenyl)hafnium dichloride (Me2Si(Ind)(2)HfCl2)-based catalyst produced highest molecular weighs polyethylene among indenyl-based catalysts. Hydrogenation of the indenyl ligand resulted in the decrease in activity and copolymerization reactivity, presumably due to the increased mobility of the ligand framework at high temperature. Diphenylmethylidene(cyclopentadienyl)(fluorenyl)hafnium dichloride (Ph2C(Cp)(Flu)HfCl2)-based catalyst produced higher molecular weight polyethylene than zirconium analog and indenyl-based hafnocene catalysts, but the activity was drastically dependent upon the alkylaluminum compound. This phenomenon was not observed in the corresponding zirconium catalyst. A broad chemical composition distribution, which was observed in Et/1-hexene copolymers obtained with Ph2C(Cp)(Flu)HfCl2-based catalyst, was attributed to the small amount of zirconium contamination.