The effect of [Al]/[Zr] mol ratio and temperature on the cocatalytic effects of tetraisobutyldialuminoxane (TIBDAO) and methylaluminoxane (MAO) for ethylene polymerization using Cp(2)ZrCl(2) catalyst were studied. The decay type kinetic profile was observed for both TIBDAO and MAO cocatalyzed ethylene polymerizations. Catalytic activity and rate of polymerization were found to be low for TIBDAO cocatalyzed ethylene polymerization when compared to MAO cocatalyzed ethylene polymerization. The differences in catalytic activity and rate of polymerization for ethylene polymerization catalyzed by Cp(2)ZrCl(2)-TIBDAO and Cp(2)ZrCl(2)-MAO were discussed with respect to the structures of MAO and TIBDAO. An active species for Cp(2)ZrCl(2)-MAO and Cp(2)ZrCl(2)-TIBDAO catalyzed ethylene polymerizations was also discussed. The polyethylene was characterized by intrinsic viscosity measurements.