This study is devoted to investigate the effects of wood fiber content on structure and properties of wood fiber /poly(lactic acid) bio-composites (WF/PLA bio-composites). The WF/PLA bio-composites with WF from 30% to 60% and from 70% to 90% were prepared using melt extrusion and high-speed mixing methods respectively. The physical, mechanical, thermal, and macromolecular properties of WF/PLA bio-composites were evaluated. The results are as follows: Density of WF/PLA bio-composites increased when WF increased from 30% to 60%, and decreased when WF increased from 70% to 90%. Water resistance of bio-composite decreased with increasing WF content. The flexural strength of WF/PLA bio-composites were lowered when adding WF into PLA. The flexural strength decreased when WF increased from 30% to 50%, and then increased when WF increased from 60% to 80%. The flexural modulus of the bio-composite was higher than that of PLA without WF. Both n and w of PLA in WF/PLA bio-composite were decreased more than 70% with WF increased. The Tg, Tm values of PLA in the bio-composites were decreased with the WF increased. The cold crystal temperature (Tcc) was decreased with WF increased at low WF content. All the property changes were related to the decreasing of molecular weight of PLA in WF/PLA bio-composites.