Ethylene/norbornene copolymerizations were conducted with [Ph_2C(Flu)(3-RCp)]ZrCl_2 [R: Me (1) or Me_3Si (2)], which give alternating EN copolymers. The activity of 1 in the absence of R_3Al was approximately twice that of 2 and increased further upon addition of R_3Al. Et_3Al increased the activity most effectively, and the M^^-_n of the produced polymer decreased from 100 000 to 25 000 g.mol^<-1>. On the other hand, the M^^-_n value increased upon addition of (Bu_3Al)-Bu-i from 100 000 to 209 000 g.mol^<-1> accompanied by an approximate to 2.5-fold increase of activity. Consequently, 1 was found to be one of the most promising complexes for the synthesis of alternating EN copolymers, of which M^^-_n value was efficiently controlled by the kind and the amount of R_3Al added.