Ethylene polymerization by zirconocene-B(C 6 F 5 ) 3 catalysts with various aluminum compounds has been investigated. It is found that the catalytic activity depended on zirconocenes used, and especially on the type of aluminum compounds. For Et(H 4 Ind) 2 ZrCl 2 (H 4 Ind : tetrahydroindenyl), the activity decreases in the following order: Me 3 Al ú i-Bu 3 Al ú Et 3 Al ӷ Et 2 AlCl. While for Cp 2 ZrCl 2 (Cp : cyclopentadienyl), it varies as follows: i-Bu 3 Al ú Me 3 Al ӷ Et 3 Al. Furthermore, the activity is significantly affected by the addition mode of the catalytic components, which may imply that the formation of active centers is associated with an existing concentration of catalytic components. Results of thermal behavior of polyethylene (PE) studied by differential scanning calorimetry (DSC) show that crystallinity of the polymer prepared with Et 3 Al is higher than that with Me 3 Al or i-Bu 3 Al. It is also found that the numberaverage molecular weight ( M V ) of the polymers prepared with Me 3 Al or i-Bu 3 Al is much higher than that with Et 3 Al. 1 H-NMR studies substantiate that i-Bu 3 Al is a more efficient alkylation agent of Cp 2 ZrCl 2 in comparison with Me 3 Al.