The catalytic properties of a series of Fe(LI) diimine complexes (diimine = N, N'-o-phenylenebis(salicylideneaminato), N, N'-ethylenebis(salicylideneaminato), N,N'-o-phenylenebisbenzal, N, N'-ethylenebisbenzal) in combination with ethylaluminoxane (EAO) for ethylene oligomerization have been investigated. Treatment of the iron(II) complexes with EAO in toluene generates active catalytic systems in situ that oligomerize ethylene to low-carbon olefins. The effects of reaction temperature, ratios of Al/Fe and reaction periods on catalytic activity and product distribution have been studied. The activity of complex FeCl2(PhCH= o-NC6H4N=CHPh) with EAO at 200 degrees C is 1.35 x 10(5) g oligomers/mol Fe (.) h, and the selectivity of C4-10 olefins is 84.8%.