The zirconadihydrophosphetes showed high activities for ethylene polymerization as well as copolymerization with α-olefins or norbornene in the presence of methylaluminoxane (MAO) as cocatalyst. The alkylated intermediates as the active species were confirmed by NMR measurement. The cause for their high catalytic activities is assumed to the disassociation and coordination of the ethylene-bridged phosphine to the zirconium species. A plausible mechanism for the polymerization is proposed.