The catalytic properties of a series of Co(II) diimine complexes Co(N<^>N)Cl-2 (A: N<^>N = N,N'-o-phenylenebisbenzal; B: N<^>N = N,N'-ethylenebisbenzal; C: N<^>N = N,N'-o-phenylenebis(diphenylmethylene)) in combination with ethylaluminoxane (EAO) as cocatalyst for ethylene oligomerization have been investigated. Treatment of the cobalt(II) diimine complexes with EAO in toluene generated active catalysts in situ that are capable of oligomerizing ethylene to low-carbon olefins. The catalytic activity and product distribution were affected by reaction conditions, such as reaction temperature, the ratios of Al/Co and the reaction time. The activity of 1.30 x 10(5) g oligomers/mol Co h for the catalytic system of CoCl2(Ph2C=o-NC6H4N=CPh2) with EAO at 200 degreesC was observed, with the selectivity of 94.4% to C4-10 olefins and 87.2% to C4-10 linear alpha -olefins.