Crystallization dramatically influences polymer rheology. Indeed, for semi crystalline polymers, “solidification” often takes place because of the increase of crystallinity rather than for temperature decrease. In this work an isotactic polypropylene, well characterized in terms of crystallization kinetics in both quiescent and flow conditions, was subjected to short steps of shear in isothermal conditions, and the evolution of viscosity was then followed during time. It was found that, for the same shear rate, an increase of the duration of the shear step induced an anticipated hardening of the material, caused by a faster crystallization. A model for flow-induced crystallization was applied to describe the crystallinity evolution during the tests, and a model for the effect of crystallinity on viscosity was applied to describe the hardening. On the basis of the results obtained, it was noticed that the effect of crystallinity on viscosity is largely determined by the volume occupation of crystalline structure, whereas the size of the spherulites seems to be much less significant.