Poly(lactic acid) (PLA) films filled with up to 50 wt % softwood flour were prepared by melt compounding and thermocompression. Thermal annealing of the melt was performed at temperatures from 90 degrees C to 120 degrees C, for 45 min. Responses on polymer-filler interactions, viscoelastic properties, crystallinity of PLA as well as PLA-wood flour-filled films were investigated by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and scanning electron microscopy (SEM). The effectiveness of fillers on the storage moduli (C) was also calculated. The results reveal that wood flour (WF) in conjunction with thermal annealing affected the melting behavior of PLA matrix, and the glass transition temperature. It was further found that the effectiveness of the wood filler in biocomposites widely improved with thermal annealing as well as with higher WF concentration. Finally, it was found that the compatibility between WF and the PLA matrix can be improved when suitable annealing conditions are applied. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 1534-1540, 2010