Random copolymerization of trimethylene carbonate (TMC) with e-caprolactone (CL) catalyzed by rare earth chloride-epoxide or rare earth isopropoxide has been investigated for the first time. It was found that in the presence of epoxide, rare earth chlorides have high activities for the copolymerization, giving high-molecularweight random copolymer P(CL-co-TMC) with a narrow molecular weight distribution. Light rare earth chloride-propylene oxide systems are more effective for the copolymerization than heavy rare earth chloride-propylene oxide systems. For the rare earth chloride-epoxide catalyst system, epoxide is the requisite component, and its amount affects the catalytic activity; while rare earth isopropoxide can catalyze the copolymerization alone. The preparative conditions of the copolymer with NdCl 3 -5PO system were studied. The reactivity ratios of CL and TMC copolymerization with NdCl 3 -5PO determined by Fineman-Ross method are 1.60 for r TMC and 0.72 for r CL , respectively. The copolymers were characterized by 1 H-and 13 C-NMR, GPC, and DSC. The mechanism study shows that the rare earth alkoxide is the active species that initiates the ring opening copolymerization of CL and TMC with acyl-oxygen bond cleavages of the monomers.