A series of nickel complexes ligated by N-((pyridin-2-yl)methylene)quinolin-8-amine derivatives were synthesized by one-pot reaction of 8-aminoquinolines, 1-(pyridine-2-yl) ketones, and nickel halides and characterized by elemental and spectroscopic analyses along with X-ray diffraction analyses. These nickel complexes exhibit two kinds of structures, namely, dimeric with octahedral-coordinated geometry and monomeric with distorted trigonal-bipyramidal geometry, in their solid states. Activated by Et2AlCl, these Ni(II) complexes exhibited good to high catalytic activities for ethylene oligomerization, and activity up to 4.9 × 107 g mol-1(Ni) h-1 was observed in the catalytic system with Cy3P as auxiliary ligand. The steric and electronic influence of the substituents as well as the influence of different catalytic conditions was carefully investigated. In particular, the correlations between reactivity and auxiliary R3P ligands were examined.