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Polylactic acid (PLA) is a kind of biodegradable polymer that has applications in a wide range of biomedical fields and food?contact packaging. However, the mechanical disadvantage limits the application of PLA?based material. The present study modified the pure PLA by orientation and investigated its mechanical and antibacterial properties. The crystallinity and the tensile strength were both drawing ratio dependent. They increased along with the draw ratio to a maximum value of 600% and then decreased. The antibacterial effect of PLA against Staphylococcus aureus and Vibrio paraemolyicus both first increased and then decreased along with the drawing ratio, reaching their highest effect at the drawing ratio of 600%. The inhibition effect of PLA reduced generally with the aging days. According to SEM images, the grape?like clusters structure of S. aureus was hardly observed and the cell structure damaged on the surface of PLA?600%, while significant changes of the morphology of V. paraemolyicus were not observed. POLYM. ENG. SCI., 2019. © 2019 Society of Plastics Engineers