The study of the crystallization process of biodegradable polymers is crucial to predict the final properties of the industrial plastic parts. In this work, the effect of the application of low shear rates on the polymorphous crystallization of the Poly (lactic acid), PLA, has been studied. Results in terms of nucleation and growth rate of crystalline structures in the spherulitic regime revealed a non-monotonous dependence of the growth rate upon the shear rate. It was also observed that in the range 110°C-130°C on increasing the shear rate a transition from alpha phase to alpha' phase and then again to alpha phase takes place. An interpretation of this peculiar behavior in terms of increase of undercooling induced by flow is suggested.