Maleic anhydride (MA) grafted poly(lactic acid) (PLA) coupling agents (MAPLA) were prepared by reactive processing. The amount of peroxide initiator and MA was changed in a relatively wide range. Coupling efficiency was checked in PLA/wood composites as a function of grafting degree, coupling agent and wood content. The analysis of the results showed that chain scission takes place in PLA during reactive modification. The occurrence of grafting could not be proved by FTIR spectroscopy, but a detailed NMR analysis showed that the degree of grafting depends on the amount of both reactants; a maximum of 2.5 MA groups/PLA chain could be grafted under the conditions used in the study. The functionalized polymer proved to be an efficient coupling agent in PLA/wood composites. Efficiency increased with increasing number of functionality and coupling agent amount. Coupling resulted in increased strength and reinforcement.Acoustic emission analysis of deformation processes supported by microscopy proved that the dominating local deformation process is the fracture of the fibers, but small extent of debonding also occurs in neat, uncoupled composites. The prevention of debonding by coupling resulted in the improved performance of the composites. Local processes initiate the immediate failure of the composite irrespectively of their mechanism.