To prepare biomass-based composites in an environmentally benign manner, vapor-phase assisted surface polymerization (VASP) was applied to prepare the composites from wood flour and poly(l-lactic acid) (PLLA) without solvent. VASP of l,l-lactide successfully proceeded on the wood flour surfaces, resulting in surface coverage by newly generated PLLA. For obtained PLLA/wood flour composites, it was clarified that grafting of PLLA on wood flour surfaces had occurred to form covalently bonded composites, with the accumulated PLLA layers having crystallized in situ during VASP. Resulting PLLA layers showed very high crystallinity of 79.2% and a high melting point close to the equilibrium melting point. Moreover, thermal degradation behavior of the composites suggested a cooperative degradation manner of the components.