The biodegradable poly(L-lactide-co-trimethylene carbonate) [poly(LLA-co-TMC)] was synthesized by molten ring-open copolymerization and the structure and properties of the copolymer were investigated by GPC, NMR, FTIR, DSC, and Instron testing. The copolymer with a higher LLA content had a higher initial molecular weight (M-n), but a lower final M-n. In contrast, the copolymer with a higher TMC content had a lower initial M-n, but its M-n increased with increasing reaction time and TMC content. The glass transition temperature (T-g), the melting temperature (T-m) and the crystallinity of the copolymer decreased with increasing TMC content. The elongation of the copolymer significantly increased with increasing TMC content while the toughness passed through a maximum.