Abstract: A new type of bio-composite consisting of poplar wood fibers and poly(lactic acid)(PLA)was prepared by hot-pressing. The PLA-based bio-composites and sole PLA were treated with cold crystallization at different heating rates. The effects of cold crystallization on performance of both materials were analyzed with the differential scanning calorimeter(DSC), polarizing microscope(POM)and scanning electron microscope(SEM). The results showed that the crystallinities of sole PLA panels and PLA-based bio-composites increased from 6% to 50%, and from 18.3% to 50%, respectively. Mechanical properties were tested with a universal testing machine and the increased crystallinity did not induce variations of the density and impact strength of both materials. However, the flexural strength and flexural modulus of wood fiber/PLA composites decreased by 30% and 16%, respectively. The flexural strength of sole PLA panels also decreased to some extent. Confirmed by SEM, the compromised mechanical strength and stiffness could be due to the poor interface compatibility between wood fibers and the polymer matrix. The process of cold crystallization was observed by POM, in which a large number of crystals formed in both materials, resulting in the worse transparency of both materials. Based on the results, transparency of PLA based composite could be adjusted using the regulation of the PLA crystallinity in the composite system.