AbstractPolylactide (PLA)‐based wood–plastic composites (WPCs) were successfully manufactured by extrusion blending followed by injection molding. The effects of polyhydroxyanoates (PHAs) on the mechanical and thermal properties and the morphologies of the PLA‐based WPCs were investigated with mechanical testing, thermal analysis, and scanning electronic microscopy (SEM). The inclusion of PHAs in the PLA‐based WPCs produced an increase in the impact resistance and a decrease in the tensile strength. The brittle–ductile transition of the impact strength for the PLA‐based WPCs toughened with PHAs was confirmed when the wood flour content was between 15 and 35 wt %. SEM images showed that the fracture surfaces of the PLA‐based WPCs toughened with PHAs were rougher than that of their nontoughened counterparts. The ternary PLA‐based WPCs exhibited ductile fracture during mechanical testing. Differential scanning calorimetry (DSC) showed that addition of PHAs into the composites caused deviations of the cold crystallization temperature and melting temperature of PLA. Thermogravimetric analysis indicated that the PHAs reduced the thermal stability of the PLA‐based WPCs. PHAs can be a green toughening agent for PLA‐based WPCs. The specific properties evidenced by the biocomposites may hint at their potential application, for example, in the automotive industry and civil engineering. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012