A telechelic poly([RS]-3-hydroxybutyrate) ([RS]-PHB) was synthesized by the ring-opening polymerization of [RS]-beta-butyrolactone (PBL) in the presence of 1,4-butanediol (BD) with distannoxane as the catalyst. This polymer was found to be terminated by secondary hydroxyl groups, having an (oxytetramethylen)oxy group in the main chain. It was subjected to block copolymerization with L-lactide by the catalysis of tin (II) octoate to obtain an A-B-A type triblock copolyester comprising poly(L-lactide) (A) and [RS]-PHB (B). In this block copolymerization, the molecular weight and the unit composition of the produced copolymer were successfully controlled by changing the lactide/[RS]-PHB ratio in feed. its: characterization revealed that this block copolymer has high potential for use as biodegradable thermoplastic elastomer.