Effect of substituents on cyclopentadienyl group for catalytic activity in 1-hexene and 1-octene polymerization with the series of Cp'TiCl2(O-2,6-(Pr2C6H3)-Pr-i) [Cp' = Cp (1a), (BuC5H4)-Bu-t (2a), 1,3-Me2C5H3 (3a), 1,3-(Bu2C5H3)-Bu-t (4a), and C5M5, (5a)]-methylaluminoxane (MAO) catalysts have been explored, and the activity increased in the order: 4a (26 kg polymer/mol Ti.h) < 1a (63) < 2a (89) < 3a (184) < 5a (728). The catalytic activity in l-hexene polymerization with the series of Cp*TiCl2(OAr) [OAr = 2,6-(Pr2C6H3)-Pr-i (5a), 2,4,6-Me3C6H2 (5b), 2,6-Me2C6H3 (5c), 2-Bu-t-4,6-Me2C6H2 (5d), 4-MeC6H4 (5e)]-MAO catalysts increased in the order: 5a (728 kg polymer/mol Ti.h) > 5d (694) > 5c (76) > 5e (48) > 5b (39). These orders are somewhat different from those in ethylene polymerization, and these differences observed here would be due to the steric bulk of monomer used as well as of substituents on both cyclopentadienyl and aryloxy groups. Although (1,3-(Bu2C5H3)-Bu-t)TiCl2(O-2,6-(Pr2C6H3)-Pr-i) (4a) showed the lowest catalytic activity for polymerization of both l-hexene and 1-octene, 4a exhibited the significant activity for copolymerization of ethylene with I-hexene, resulting in obtaining copolymer with relatively high I-hexene contents (20.2-36.5 mol%) with relatively narrow molecular weight distributions.