Polymerizations of ethylene have been carried out by using Cp-2*Zr(NMe2)(2) (CP* = C5Me5) compound combined with common alkyl aluminums (AIR(3)) and methylaluminoxane (MAO) as cocatalysts. The AIMe(3) cocatalyzed system showed no activity due to the formation of stable but inactive heterodinuclear [Cp-2*2Zr( mu-Me)(2)AlMe2](+) cations; however, the bulkier AlR3 [AlEt3, Al(i-Bu)(3) and Al(i-Bu)(2)H] cocatalyzed systems showed very high activities. Especially, Cp-2*Zr(NMe2)(2)/Al(i-Bu)(3) catalyst showed higher catalytic activity and produced higher molecular weight (MW) polymer than Cp-2*Zr(NMe2)(2)/MAO catalyst, demonstrating both MAO and bulky AlR3 are effective cocatalysts for Cp-2*Zr(NMe2)(2) compound. (C) 2003 Published by Elsevier B.V.