A novel nickel complex ligated with 2-(2,6-diisopropylanilino)-1,4-naphthoquinone (1) was synthesized. The molecular structure of 1 determined by X-ray analysis was a square-planar geometry. Complex 1 conducted ethylene polymerization at 40 degrees C in a low activity to give linear polyethylene. On the other hand, 1 activated with 4 eq. of B (C6F5)(3) was highly active for ethylene polymerization and gave a polymer possessing short chain branches of methyl, ethyl and propyl groups formed by a chain walking mechanism, as well as long chain branches, of which the content was almost the same as the total content of short chain branches. These results suggest that the macromonomer formed via beta-hydride elimination should have effectively copolymerized with ethylene to give the long chain branches in the B(C6F5)(3)-activated system.