The structural transitions and free volume properties for two kinds of linear polyurethane prepared with polybutalene adipate polyol and polycaprolactone polyol (PBU and PCU) respectively were studied by means of positron annihilation spectroscopy in the temperature range from 140 K to 360 K

DSC and WAXD measurements．The experimental results indicated that there were three structural transition temperatures for these two polyurethanes (PUs) from 140 K to 360 K．The transition at low temperature (at around 200 K) was attributed to the glass transition temperature (Tg) of polyester soft segments existing in PU and the transition at 275K could be related to the small amount water adsorbed in PU samples．The high temperature transition (at about 310 K) corresponded to the melting point of soft segment crystals in PBU and was also related to the Tg for compatible regions of the disordered hard segments with some soft segments existed in PCU．When the temperature was lower than the Tg of soft segments in PU

both the size and the relative concentration of free volume increased with increasing temperature for PBU and PCU．While the temperature was higher than the Tg of soft segmensts but lower than 275K

the relative concentration of free volume reached a constant value but the size of free volume increased further．When the temperature was higher than 275 K but lower than the melting point of soft segment crystals or the Tg of hard／soft segment miscible regions

the relative concentration of free volume still kept constant but the size of free volume increased appreciably．When the temperature increased more

both the size and the relative concentration of free volume existing in PUs increased again．The distributions of free volume were also examined at various temperatures for these two PUs．All these experimental results could be related to the movement of macromolecular chains and the morphologies for these PUs measured by using DSC and WAXD．