Polycaprolactone(PCL) was synthesized via the ring-opening polymerization of E caprolactone( epsilon-CL) with lanthanide tris(2.6 - dimethylphenolate)s[Ln(ODMP)(3)] as initiator under mild conditions for the first time. The effects of reaction conditions on the epsilon -CL polymerization are discussed. Kinetic studies indicate that the polymerization rate is first order with respect to both monomer concentration and catalyst concentration, respectively, and the overall reaction activation energy amounts to 62.3kJ/mol. H-1 NMR end group analysis on the polymer demonstrates the ring-opening polymerization of epsilon -CL proceeds according to coordination-insertion mechanism with acyl-oxygen bond cleavage of the monomer.