Ethylene polymerization have been carried out by using stereorigid diamide rac-Me₂Si(CMB)₂Zr(NMe₂)₂(rac-1,CMB=1-C₅H₂-2-Me-4-^{t/Bu) compound in the presence of methylaluminoxane(MAO) or AIR₃/[CPh₃][B(C₆F₅)₄] (2). Both catalytic systems are highly active and their activities are strongly dependent on the polymerization temperature (T_p). The polymerizaion especially at low T_p results in a precipitation of polymer during the initial stage of polymerizaion and a change of polymerization medium to heterogeneous phase. The curved Arrhenius plot of activity and the dissimilarity of the Arrhenius plot of initial rate of polymerizaion from the Arrhenius plot of the rate of long-time polymerization indicate the present catalytic systems are experiencing mass transfer limitations from the early period of polymerizaion. The rac-1/AIR₃/2 system also shows very high activity, when AIR₃ is A(i-Bu)₃,Al(i-Bu)₂H, and AlEt₃, but the rac-1/AlMe₃/2 system shows very low activity. The molecular weight(MW) of polymer decreases sharply as T_p increases, and the high MW of polymers formed especially at low T_p causes the precipitation of polymers.}