Various (arylimido)(aryloxo)vanadium(V) complexes of the type VCl2(NAr)(OAr') [NAr = N-2,6-Me2C6H3 (2), N-2,6-(Pr2C6H3)-Pr-i (3); OAr'= O-2,6-Me2C6H3 (a), O-2,6-(Pr2C6H3)-Pr-i (b), O-2,6(t)Bu(2)-4-MeC6H2 (c)] have been prepared in high yields from VCl3(NAr) with Ar'OH in n-hexane. 2b exhibited remarkable catalytic activity for ethylene polymerization in the presence of methylaluminoxane (MAO), affording high molecular weight polymer with unimodal molecular weight distribution (M-w = 2.92 x 10(6), M-w/M-n = 2.88). V(CH2Ph)(2)(N-2,6-Me2C6H3)(O-2,6-(Pr2C6H3)-Pr-i) (4) initiated ring-opening metathesis polymerization (ROMP) of norbornene without cocatalyst, affording high molecular weight polymer with unimodal molecular weight distribution (M-w = 4.69 x 10(6), M-w/M-n = 1.93). The ROMP also proceeded efficiently if 2b or 2c was treated with AlMe3 (5-10 equiv) in toluene, and high stereoregularity (trans/cis = 91/9) in the olefinic double bond for the resultant polymer was observed by 2b. On the other hand, remarkable catalytic activities by 2b (3700-8300 kg of PE/mol of V h) have also been observed for ethylene polymerization in the presence of Et2AlCl, which was an ineffective cocatalyst for the above ROMP.