Ring-opening polymerization of ε-caprolactone has been carried out by using rare earth Schiff base complexes: lanthanide tris(N-phenyl-3,5-di-t-butylsalicylaldiminato)s [Ln(OPBS)3] as single component catalyst for the first time. The influences of different rare earth elements, monomer and catalyst concentration as well as reaction time on the polymerization were investigated. Mechanism studies showed that monomer inserts into the active site with the acyl-oxygen bond scission rather than the break of alkyl-oxygen bond.