kulicke et al. described the possible mechanism of ultrasonic degradation. it appears that the chain scission, due to elongational flow fields between collapsing cavitation bubbles, always occures close to the center of grvity of mloecule. that is why no monomer is formed and no side reaction occur during ultrasonic degradation. this study aims to investigate the molecular weight degradation kinetics and disribution change of SPG under various ultrasonic treatment parameters. ethanol fractional precipitation technology was used to gain the fraction with different molecular weight. the rheological property of USPG sample and fractions were futher determined to provide theoretical basis for the ultrasonc degradation