A series of Ni(II) complexes 4a–f ligated by the unsymmetrical phosphino-oxazolines (PHOX) were synthesized and characterized by elemental analysis and IR spectroscopy, and the structures of complexes 4c–4e were confirmed by the X-ray crystallographic analysis. All derivatives showed distorted tetrahedron geometry by the nickel center and coordinative atoms. Upon activation with methylaluminoxane (MAO) or Et2AlCl, these complexes exhibited considerable to high activity of ethylene oligomerization. The ligands environments and reaction conditions significantly affect their catalytic activities, while the highest oligomerization activity (up to 1.18×106 g·mol−1(Ni)·h−1) was observed for 4d at 20atm of ethylene. Incorporation of 2–4 equivalents of PPh3 as auxiliary ligands in the 4a–f/MAO catalytic systems led to higher activity and longer catalytic lifetime.