A series of poly(R-3-hydroxybutyrate)/poly (epsilon -caprolactone)/1,6-hexamethylene diisocyanate-segmented poly(ester-urethanes), having different compositions and different block lengths, were synthesized by one-step solution polymerization. The molecular weight of poly(R-3-hydroxybutyrate)-diol, PHB-diol, hard segments was in the range of 2100-4400 and poly(epsilon -caprolactone)-diol, PCL-diol, soft segments in the range of 1080-5800. The materials obtained were investigated by using differential scanning calorimetry, wide angle X-ray diffraction and mechanical measurements. All poly(ester-urethanes) investigated were semicrystalline with T-m varying within 126-148 degreesC. DSC results showed that T-g are shifted to higher temperature with increasing content of PHB hard segments and decreasing molecular weight of PCL soft segments. This indicates partial compatibility of the two phases. In poly(ester-urethanes) made from PCL soft segments of molecular weight (M-n greater than or equal to 2200), a PCL crystalline phase, in addition to the PHB crystalline phase, was observed. As for the mechanical tensile properties of poly(ester-urethane) cast films, it was found that the ultimate strength and the elongation at the breakpoint decrease with increasing PHB hard segment content.