Ultra-thin LLDPE parts were prepared by microinjection molding. Mechanical properties and microstructure of samples before and after annealing at different temperatures (60, 80, 100 degrees C) were analyzed. Tensile test indicates that toughness has been enhanced at least 7 times after being annealed in parallel with significant increase of tensile strength. Microstructure characterizations, including differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), scanning electronic microscope (SEM) and polarized Fourier transform infrared spectroscopy (FTIR) were performed to investigate the variations of microstructure and to further establish the relationship between microstructure and mechanical properties. It is suggested that the increased crystallinity and perfection of crystals upon annealing are beneficial to the reinforcement. Formation of a stronger physical cross-linking network due to the increase of connecting points is in favor of the enhanced strength as well. Moreover, the notable improvement of toughness in the annealed ultra-thin LLDPE parts is ascribed to combined effects of microvoids developed in both the annealing and deformation process. This work offers a simple approach to prepare ultra-thin LLDPE parts with excellent toughness and strength. (C) 2013 Elsevier Ltd. All rights reserved.