In this study, wood flours (WFs) were modified by sodium-montmorillonite (Na-MMT) and didecyl dimethyl ammonium chloride (DDAC) in two steps to form organo-montmorillonite (OMMT) inside WFs at different OMMT concentrations (0.25, 0.5, 0.75 and 1%, respectively). The surface properties of WFs were investigated as an approach to understand the compatibility of WF/polypropylene (PP) composites. The mechanical properties of the composites were also tested. The results showed that: (1) OMMT modification did not influence the surface morphologies of WFs; (2) owing to the covering of OMMT on WF surface, with increasing OMMT concentration, the surface free energies of WFs increased with both increases of dispersive and polar components, and the surface hydroxyl groups of WF decreased, suggesting good compatibility with non-polar PP; (3) the flexural and tensile strength were correlated with the dispersive component fraction. Significant improvements in flexural and tensile strength were found at OMMT concentration of 0.5%. With increase of OMMT concentration, the flexural and Young's modulus increased. However, the impact strength decreased. (C) 2015 Elsevier B.V. All rights reserved.