Two types of salicylaldiminato-based nickel complexes, mono-ligated Ni(II) complexes ([O-C6H4-o-C(H)=N-Ar]Ni(PPh3)(Ph) (5), [O-(3,5-Br2)C6H2-o-C(H)=N-Ar]Ni(PPh3)(Ph) (6), [O-(3-t-Bu)C6H3-o-C(H)=N-Ar] Ni(PPh3)(Ph) (7)) and bis-ligated Ni(II) complexes ([O-(3,5-Br2)C6H2-o-C(H)=N-Ar]2Ni (8), [O-(3,5-Br2)C6H2-o-C(H)=N-2-C6H4(PhO)]2Ni (9), Ar = 2,6-C6H3(i-Pr)2) were synthesized and characterized by Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), mass spectrography (MS) and elemental analysis (EA). In the presence of methylaluminoxane (MAO) as cocatalyst, all the nickel complexes exhibited high activities for the polymerization of methyl methacrylate (MMA) and syndiotactic-rich poly(methyl methacrylate) (PMMA) was obtained. The complexes with less bulky substituents on salicylaldiminato framework possessed higher activities, while with the same salicylaldiminato, the mono-ligated nickel complexes showed higher catalytic activity than bis-ligated ones. Keywords late transition metal catalyst, methyl methacrylate, polymerization, salicylaldiminato nickel complexes, methylaluminoxane, syndiotactic structu