Ni(II)-alpha-diimine catalyst [(2,6-i-Pr)(2)C(6)H(3)-DAB(An)]NiBr(2) plus methylaluminoxane was successfully used in the homopolymerization of ethylene, 1-hexene, and 1-octene and the copolymerization of ethylene with 1-hexene and 1-octene in n-hexane. The polymerization of 1-octene was conducted in a controlled manner with a narrow molecular weight distribution (M (w)/M (n) = 1.2-1.5) and with the weight-average molecular weight increasing linearly with the monomer conversion. The molecular weights, T (g), T (m), branching degree, and density of the obtained (co)polymers were greatly controlled by ethylene pressure and polymerization temperature. Compared with that of ethylene homopolymer, the branching degree of the copolymers prepared by the copolymerization of ethylene with 1-hexene or 1-octene increased, whereas the molecular weight, density, T (m), and catalyst activity decreased. However, compared with those of the homopolymer of 1-hexene or 1-octene, the copolymer density, T (m), and catalyst activity increased, whereas the branching degree declined.