Poly (lactic acid) (PLA), a biodegradable aliphatic semi-crystalline polyester was filled with 40 wt% spruce wood flour (WF), to produce composite materials Hydrothermal treatment, as well as maim anhydride, vinyltrimethoxysilane, and stearic acid surface treatments were applied The influence of surface modifications for WF was tested in terms of thermal, mechanical, and viscoelastic properties The recorded results show that in both, the untreated and treated PLA/WF composites, the rigid amorphous phase content has been enhanced The presence of WF causes a stiffness increase of the PLA/WF composites, while damping factor was decreased The effect of wood surface modifications on interfacial compatibility with PLA was estimated by dynamic fragility parameter m calculated according the Williams-Landel-Ferry equation The incorporation of untreated WF increased dynamic fragility of PLA/WF composites markedly, whereas used Wane, maim anhydride and hydrothermal treatments lead to lower values of parameter m POLYM ENG SCI, 51143-150, 2011 (C) 2010 society of Piastics Engineers