The oligomerization of ethylene catalyzed by Cp 2 ZrL 2 (L=Cl, Me, OC 6 H 4 -Me-p) with ethylaluminoxane or Et 3 Al as cocatalyst (Al/Zr=100, 150 o C, P(C 2 H 4 )=1.4 MPa) afforded not only common chain oligomers but also a cyclic oligomer, methylenecyclopentane. The selectivity of methylenecyclopentane reached 39% under optimal conditions. The addition of C 5 H 5 N to the catalytic system of Cp 2 ZrCl 2 /Et 3 Al was capable of further improving the selectivity of methylenecyclopentane to 43%.