Effects of structural variations of the diimine ligand on catalyst activities for vinyl polymerization of norbornene (NB) have been investigated by a series of Ni(II) α-diimine catalysts of the general formula: [{ArN=C(Ac)-C(Ac)=NAr}]NiBr2 (Ac=acenaphthyl) (Cat(H), Ar=C6H5; Cat(2,6-Me), Ar=2,6-C6H3Me2; Cat(2,6-Et), Ar=2,6-C6H3Et2; Cat(2,6-iPr), Ar=2,6-C6H3 i-Pr2; Cat(2,3-Me), Ar=2,3-C6H3Me2; Cat(2,4-Me), Ar=2,4-C6H3Me2; Cat(2,5-Me), Ar=2,5-C6H3Me2; Cat(3,5-Me), Ar=3,5-C6H3Me2; Cat(2,4,6-Me), Ar=2,4,6-C6H2Me3). In situ reactions with methylaluminoxane generated the active catalysts, and they showed good activity towards NB polymerizations. As indicated by relatively higher activities of Cat(H) and Cat(3,5-Me), it can be generalized that catalysts having 2,6-substituents are less active due to steric interaction between monomer and substituents. In addition, electron donating methyl groups at 2-, 4- or 6-position on the N-aryl have a con effect and that at 3,5-position has a pro effect.