Poly((L)-lactic acid) (PLLA) sheets with dramatic improvement of mechanical properties, especially toughness, were obtained by uniaxial pre-stretching around T-g. It was found that a network structure consisting of cohesional entanglements was formed during the usual compression molding process, leading to the brittleness of PLLA, and the destruction of the network structure due to disentanglement after pre-stretching, leading to the toughness of PLLA. The network structure could not be destroyed when pre-stretching ratio (PSR) was in the elastic deformation region I and strain softening region II (PSR < 0.2). However, when PSR got into the strain hardening region III (PSR = 0.2), the network structure was destroyed resulting from the disentanglement, leading to the brittle to ductile transition. With PSR increasing from 0.2 to 0.4, the orientation was formed in pre-stretched PLLA, leading to further increase of the elongation at break. And then the degree of orientation increased along with the occurrence of the mesophase (PSR >= 0.5), therefore, the modulus and strength increased, while the elongation at break decreased. However, the elongation at break was still larger than that of undrawn PLLA. (C) 2018 Elsevier Ltd. All rights reserved.