A series of zirconium salen-type complexes were prepared by using ZrCl4 and sodium salts of the tetradentate Schiff base ligands. In combination with Et2AlCl, they displayed moderate to high catalytic activities in ethylene oligomerization under a pressure of 1.0-1.8 MPa at 150 degreesC in toluene. The main products are C-4-C-10 olefins with good selectivity to linear alpha-olefins. The turnover frequency (TOF) of catalyst system Zr(salen)Cl-2 (.) THF/Et2AlCl, under a pressure of 1.8 MPa and in an Al/Zr molar ratio of 300, was 4.93 x 10(4) h(-1) with a satisfactory selectivity to C-4-C-10 olefins (89%), and linear alpha-olefins accounted for 95%. The steric and conjugate effects of the ligands on ethylene oligomerization, as well as the effects of cocatalysts, the Al/Zr molar ratios, the reaction temperature, the ethylene pressure, and the lifetime of the catalysts were studied. (C) 2003 Published by Elsevier Inc.