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 of PLA, biaxial stretching is known as an elaboration process able to improve the ductility of some other brittle polymers, for instance poly(styrene) [1]. The goal of this study is focused on understanding the influence of molecular orientation on the mechanical behavior of PLA through biaxial stretching. 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. As a result, while unstretched PLAs exhibited brittle behavior at room temperature, a remarkable enhancement of ductility (over 100%) was observed for biaxially oriented films of both grades. This study highlights for the first time that a critical molecular chain orientation of 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. Reference [1] Choi, K. J., Spruiell, J. E., & White, J. L. (1989). Polymer Engineering & Science, 29(21), 1516-1523