Syntheses of copolymers composed of optically active depsipeptides (3,6-dimethyl-2,5-morphorinedione) and L-lactide-poly(L-3,L-6-dimethyl-2,5-morphorinedione-co-L-lactide), poly(L-3,DL-6-dimethyl-2,5-morphorinedione-CO-L-lactide), and poly(L-3,D-6-dimethyl-2,5-morphorinedione-co-L-lactide)-were examined in an effort to improve the biodegradability and physical properties of homopoly(L-lactide). In degradation tests, the copolymers composed of 3,6-dimethyl-2,5-morphorinedione and lactide in the ratios 10/90 to 13/87 exhibited high biodegradability toward proteinase K, whereas a homopolymer, poly(L-lactide), exhibited very poor biodegradability (only 50% after 200 h). These polymers composed of 3,6-dimethyl-2,5-morphorinedione/L-lactide in 11/89 to 13/87 ratios also degrades rapidly after being in compost for 30 days. The resulting copolymers, however, showed relatively low elongation properties, Therefore, ternary copolymerizations Of L-3,DL-6-dimethyl-2,5-morphorinedione, E-caprolactone, and L-lactide were explored in an effort to improve their mechanical properties, especially the elongation, and sufficient results were obtained with an approximate ratio of 3/11/86.