Ring opening polymerization of ε caprolactone (Cl) catalyzed by bis(tert butyl cyclopentadienyl) methyl neodymium was studied. The yield was obviously dependent on the catalyst concentration and the polymerization temperature. The molecular distribution of the polymer changed little with the increase of polymerization time. The initiation mechanism of polymerization has been studied through oligomer end group analysis by NMR. The results demonstrate that catalyst reacts first with ε-caprolactoner to give a lanthanide alkoxide which then catalyzed caprolactone polymerization.