Novel poly(ester-urethane) block copolymer has been synthesized by use of telechelic hydroxylated poly-1(R)-3-hydroxyoctanoate] (PHO-diol) with number-average molecular weight (M-n) of 2400 as the amorphous soft segment, telechelic hydroxylated poly-[(R)-3-hydroxybutyratel (PHB-diol) with Mn of 2600 as the crystalline hard segment, and L-lysine methyl ester diisocyanate (LDI) as the junction unit. Reaction of the PHO-diol, PHB-diol, and LDI at a ratio of 1:1:2 in the presence of dibuthyltin dilaurate as catalyst afforded the block copolymer in 70% yield with a molecular weight (M.) of 33 500 (GPC) and a M-n of 10 600 (VPO). The chemical structure of the new polymer was confirmed by H-1 NMR, C-13 NMR, and IR spectra. On average, 2 PHO-diol, 2 PHB-diol, and 5 LDI were incorporated in each molecule of the block-copoly(ester-urethane). The thermoplastic proper-ties were very good with a melting temperature (T.) of 146 degreesC and the glass transition temperature (T,) of -6 degreesC (DSC). The mechanical properties, obtained from the stress/strain test, are comparable with other biocopolyester and blockcopoly(ester -urethane).