In the present work, the influence of well-defined simple shear flow histories on the isothermal crystallization of an isotactic polypropylene (I-PP) has been investigated. At first, the research of the flow conditions in terms of temperature, shear rate ((gamma) OVER DOT) and shear strain (gamma) has been performed by means of the rheological technique. The continuous shearing analysis enabled us to build the flow curve at 144degreesC showing a Newtonian region as well as a shear-thinning zone. Indeed, for (gamma) OVER DOT above a critical value, the molecular orientation occurring during flow provides a kinetic promotion of the crystallization process. In the rheological step-shear flow analysis, an increase of the flow sensitivity parameter, k(S)/k(Q), with increasing the shear rate at a constant strain (gamma = 150) is observed. The structure of the crystallized samples has been investigated by differential scanning calorimetry (DSC) and wide angle X-ray scattering (WAXS) methods. In agreement with the DSC, the WAXS results show that crystals with a certain bimodal distribution are generated in the samples crystallized under step-shear flow conditions. A small orientation of the (110) plane of the i-PP alpha-phase crystals is also detected. (C) 2005 Society of Plastics Engineers.