The damping coefficient (tan delta) of wood flour filled polypropylene composites, having varying filler concentrations were measured using the free vibration decay of disk-shaped specimen, vibrating in flexural vibration mode. The damping coefficients decreased with the increase of filler load in composites. There was no significant difference in damping behavior of composites with and Without compatiblizer at low filler level (upto 30%). At higher filler loading (>30%), composites with compatiblizer had lower damping coefficient suggesting improved interfacial adhesion between wood and polypropylene. The damping in composite is attributed to the damping because of the composite constituents and damping at the interface. The damping because of interface was estimated using a model and was found to increase with the increase in filler loading. At higher filler content, damping due to interface in composites with compatiblizer was significantly lower than in composites without compatiblizer suggesting a better interfacial adhesion between the wood filler and polypropylene matrix with compatiblizer. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114:2421-2426,2009