In this article, the influence of ammonium polyphosphate (APP) and ammonium polyphosphate modified with 3-(Methylacryloxyl) propyltrimethoxy silane (M-APP) on mechanical properties, flame retardancy, and thermal degradation of wood flour-polypropylene composites (WF/PP composites) have been investigated. Polypropylene grafted with m-isopropenyl-alpha,alpha-dimethylbenzyl-isocyanate (m-TMI-g-PP) was used to improve the adhesion of WF/PP composites. APP and M-APP were used as flame retardants. The experimental results demonstrated that addition of M-APP obviously enhanced mechanical properties of WF/PP composites. According to cone calorimetry results, M-APP is also an effective flame retardant for WF/PP composites, compared to that of APP. It was also found that M-APP decreased the 1% weight loss temperature and increased char residue. The thermal degradation of wood flour based upon the first peak temperature of wood decreased from 329.3 to 322.9 degrees C and the thermal degradation of PP based upon the second peak temperature of PP improve from 518.0 to 519.6 degrees C, when M-APP was added to the WF/PP composites. From SEM results the char layer of the 25% M-APP systems is much more intumescent than that of the 25% APP systems, indicating that 3-(Methylacryloxyl) propyltrimethoxy silane can improve the char-forming ability of WF/PP composites. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122: 849-855, 2011