Crystallization experiments after shear treatment were performed to study the effect of shear on the crystallization kinetics and morphology of polypropylene. Experiments performed with a polarized light microscope revealed an increase in the number of activated nuclei after shear treatment, and alignment of these nuclei along flow lines after long shearing times. A crystallization half time was defined using the transmitted intensity between crossed polarizers, and rheological measurements. A kinetic model valid for PP crystallization under quiescent conditions was extended to include shear-induced crystallization by linking the extra number of activated nuclei observed after shear treatment to the first normal stress difference. This model accounts for the theological behavior of the molten polymer and is then used to predict the higher sensitivity to shear treatment of long molecular weight fractions.