Abstract: Biomass-derived fillers reinforced composites have always been considered as potential substitutes for petroleum-based products. In this work, high content of bleached pulp fibers (10wt% to 50wt%) were adopted to modify polylactic acid (PLA) with maleic anhydride grafted polypropylene (MA-g-PP) as the compatibilizer. Two pre-melting processes of fiber/PLA composites were tried, i.e., roll-grinding plus extruding as well as internal mixing process. Tensile test, SEM, FTIR、DMA and TGA were employed to comprehensively investigate the effects of processing methods and fiber content on the mechanical and thermal properties, and microstructure of fiber/PLA composites. The results showed that, internal mixing process (IM process) is more effective than grinding and extruding process (GE process) to achieve good dispersion and almost oriented-arrangement of the fibers in the matrix. IM-processed composites with fiber content up to 50wt% acquired the maximum tensile strength (50.49 MPa, slightly higher than PLA) and Young's modulus (2.56 GPa, 45.8% higher than PLA). Damping capacity and loss factor of composites are reduced by the rigid filled pulp fibers, while the peak temperature is positively migrated. Pulp fibers added have no adversely effect on thermal stability and improve the residues content of PLA composites. It can be concluded that bleached pulp fiber is an effective filler for PLA-based composites.