The catalyst system DPA(Na)/NiCl2 . 6H(2)O/Zn (DPA(Na), sodium salt of diphenylphosphinoacetic acid has high activity and selectivity to form lower alpha-olefins in the oligomerization of ethylene. The amount of zinc highly affects the activity, and the Zn2+ which came from the oxidation of zinc is likely to enhance the selectivity to lower alpha-olefins. The solvent used in this system offers the hydrogen of the nickel hydride.