This work deals with the influence of biaxial orientation on the mechanical behavior of Polylactide (PLA) films. Comparisons between a crystallizable grade and a non-crystallizable one have been made in order to separate the effect of macromolecular orientation from the potential influence of the crystalline phase induced during the thermomechanical treatment. While unstretched PLAs exhibited brittle behavior at room temperature, it was observed for both types of PLA the strains at break for biaxially drawn films were remarkably enhanced to values over 100%. This study highlights for the first time, in the case of PLA, that a critical molecular chain orientation of the amorphous phase is the necessary condition to induce a ductile behavior. In-situ small-angle X-ray scattering (SAXS) experiments and post-mortem microscopic observations have revealed that it is a change of elementary plastic deformation mechanisms that is at the origin of this Brittle-to-Ductile (B-D) transition.