Poly (L-lactic acid) (PLLA) is a biobased polymer, and poly(trimethylene terephthalate) (PIT) is a semibiobased polymer. However, PLLA is biodegradable, and PTT is not. In this paper, their copolymers (PTHT-PLLA) are synthesized via chain-extension reaction of hydroxyl terminated poly(trimethylene-co-hexamethylene terephthalate) (PTHT-OH) and hydroxyl terminated poly(L-lactic acid) (PLLA-OH) using toluene-2,4-diisocyanate (TDI) as a chain extender. The structures and molecular weights of PTHT-OH, PLLA-OH, and PTHT-PLLA were characterized by Fourier transform infrared (FTIR) spectroscopy, proton nuclear magnetic resonance (H-1 NMR), and gel permeation chromatography (GPC). The thermal, crystalline, and mechanical properties of PTHT-PLLA were further studied by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and tensile testing. The resulting PTHT-PLLA copolyesters went through a two-stage thermal decomposition behavior and showed two glass transition temperatures. The tensile testing results showed that PTHT-PLLA copolyesters have excellent flexibility with a strain of more than 300%, which is much higher than that of PLLA. The copolyesters are expected to have a better biodegradability than PTT.