We investigate the simultaneous effect of pressure P and shear flow on the crystallization process of polypropylene using a modified pressure-volume-temperature measurement system that has been developed in our laboratory. We focus on the inverse of half-conversion time l/t(1/2) that is defined by the inverse of the time when the relative crystallinity of system reaches to one half. It is found that the pressure dependence of the quantity l/t(1/2) can be expressed by a shifted degree of super-cooling DeltaT(P) by pressure. We also find that l/t(1/2)(T,P,gamma over dot) under both pressure and shear flow can be described by a sum of l/t (1/2)(T, P,gamma overdot = 0) under no shear and a linearly increasing term with shear rate. (C) 2003 Published by Elsevier Ltd.