The rheology and processability of ultra high molecular weight polystyrene (UHMWPS) synthesized by rare earth catalyst were investigated by using capillary rheometer and Hakke torque rheometer．It was found that the theological behavior of UHMWPS is remarkably characterized by its unstable flow at low critical shear rate and ultrahigh melt viscosity．The unstable flow is attributed to the long relaxation time of the samples resulted from their ultrahigh molecular weight

and a dependence of the critical shear rate on the molecular weight and temperature is presented．Moreover

the low molecular weight and higher temperature are favorable to reduce the melt viscosity

to increase the critical shear rate and to improve the apparent quality of extrudates．The extreme entanglement of polymer chains results in not ouly the ultrahigh melt viscosity

but also the faster dis-entanglement process which leads to a stronger dependence of melt viscosity on shear rate．Furthermore

it is indicated that the processability of UHMWPs is inferior to that of conventional PS because of its higher balance torque determined by Hakke rheometer．