The polymerization of ethylene was studied by using a series of polysiloxane-bridged dinuclear zirconocenes [(SiMe 2 O) n SiMe 2 (C 5 H 4 ) 2 ][(C 9 H 7 )ZrCl 2 ]2 (7, n = 1; 8, n = 2; 9, n = 3), the corresponding mononuclear zirconocene (C 5 H 5 )(C 9 H 7 )ZrCl 2 , 10, and the pentamethylene-bridged dinuclear zirconocene [(CH 2 ) 5 (C 5 H 4 )2][C 9 H 7 )-ZrCl 2 ] 2 , 13. From the polymerization studies using these catalysts it was found that (i) activities of the polysiloxane dinuclear zirconocenes 7-9 were lower than that of the corresponding mononuclear zirconocene 10, (ii ) molecular weights of polyethylenes produced by the dinuclear metallocenes are greater than that of polyethylene produced by the mononuclear metallocene, (iii) the complex 9 holding the longest bridging ligand exhibited the highest activity but produced a polymer having the smallest molecular weight among the polysiloxane-bridged dinuclear zirconocenes, and (iv)the pentamethylene-bridged dinuclear metailocene 13 showed higher activity than the complexes 7-9 and the mononuclear zirconocene 10. The formation of the lowest molecular weight of polyethylene by 9 was attributed to the influence of electron withdrawal caused by the Lewis acid-base interaction between the acidic aluminum of the cocatalyst and the basic oxygen at the polysiloxane linkage as well as the lack of a steric problem. An increase in steric congestion around the metal center led to not only a decrease in catalytic activity due to preventing facile monomer access to the active site but also an increase in the molecular weight of polyethylenes due to supressing β-H elimination.