Polyurethanes containing different starch contents were synthesized in a one-step reaction by suspending starch granules in polycaprolactone diol, MDI and 1,4 butane diol in a bulk phase at 175 degreesC. The products were characterized by FTIR spectroscopy, SEM, DSC, and swelling behavior. Their mechanical properties, e.g. tensile strength and elongation, were measured for different starch contents. The starch dispersed well as a grafted state in the polyurethane phase. The grafted percentage of polyurethane to starch granules increased with the starch content to a maximum point (about 20 wt%) and then decreased due to gapping between the two phases and probably the homo-polymerization tendency of the polyurethane. The DSC indicated that T-g increased with the starch content due to the decreased average molecular weight of the homo-polyurethane. Three endothermic transitions at 60-70 degreesC (I), similar to 150 degreesC (II), 190-210 degreesC (III) were observed. Transition I was not changed by the starch content, whereas transition II appeared only for the psb2m3 series (32-48 wt% hard segment) at the lower range of 26 wt% of starch content. The temperature of transition III, which is related to the melting point of the hard segments, increased with the starch content despite a decrease in the molecular weight of the homo-polyurethane. The tensile strength and the elongation of the polymers slightly increased or were constant up to about 20 wt% of starch, and then decreased rapidly because of phase separation (gapping) between the starch granules and the polyurethane phase and division of the starch granules.