Three series of isotactic polypropylene characterized by different molecular weights and the same isotactic index have been studied during crystallization under static and shearing conditions. The shear is induced by the displacement of a glass fiber in the molten polymer. The monoclinic alpha -phase is here formed under shear with a columnar organization at the surface of the glass fiber, and does not appear under static condition. The growth-rate, constant during the shear-induced crystallization experiment, is compared with the result obtained from static crystallization. An important increase of the growth-rate due to the shear flow is observed. This increase depends on the molecular structure. The average molecular weights (M) over bar (w) and (M) over bar (z) seem to be the most important molecular parameters, for which an excellent correlation is obtained. The increase of these parameters (M) over bar (w) and (M) over bar (z) leads to a significant enhancement of the growth-rate, which can be multiplied by a factor of 10 in the present conditions.