Polylactide (PLA) is one of the most promising biopolymers that actually takes part in commercialized biopolymer market. However, its intrinsic brittleness has found to be a major limit to a wider range of applications. Among the various chemical and physical approaches reported in literature aiming at improving the mechanical properties, biaxial stretching is known as an elaboration process that can improve the ductility of some brittle polymers. In this context, the goal of this work is to study the influence of chain orientation on the mechanical behavior of Polylactide (PLA). While isotropic PLA exhibits as expected a brittle behavior upon uniaxial tension at room temperature, pre-oriented PLA samples display a ductile behaviour with a strain at break exceeding 100%. In order to better understand the origin of this Brittle to Ductile (B-D) transition, both a crystallizable grade of PLA (C-PLA) and a non-crystallizable grade (NC-PLA) were studied to be able to separate the effects induced by macromolecular orientation from the ones due to crystalline phase. Through an in-depth structural characterization of the pre-oriented films, we have demonstrated that it is the macromolecular orientation in the amorphous phase which is the key parameter governing this B-D transition. Additional structural analyses by Small-Angle X-ray Scattering (SAXS) carried out in situ during the stretching of preoriented or non-oriented samples, combined with morphological observations, have shown that the B-D transition corresponds to a change in the elementary plasticity mechanisms: crazing, predominant in the case of brittle samples, gradually evolves toward the development of shear bands. Supplementary postmortem analyses by SAXS have also shown that the macromolecular orientation has no influence on the geometry of the cracks but induces a reduction in the density of cracks formed during stretching. From these analyses, the critical crack nucleation stress (σcr) as a function of the degree of orientation has been determined. It has thus been shown that the change in deformation mechanisms results from the increase in the crack initiation stress σcr with the macromolecular orientation while the shear band initiation stress is unchanged.