Mechanical properties such as tensile and impact strength behavior of teak wood flour (TWF)-filled high-density polyethylene (HDPE) composites were evaluated at 0-0.32 volume fraction (Phi(f)) of TWF. Tensile modulus and strength initially increased up to Phi(f) = 0.09, whereas a decrease is observed with further increase in the Phi(f). Elongation-at-break and Izod impact strength decreased significantly with increase in the Phi(f). The crystallinity of HDPE also decreased with increase in the TWF concentration. The initial increase in the tensile modulus and strength was attributed to the mechanical restraint, whereas decrease in the tensile properties at Phi(f) > 0.09 was due to the predominant effect of decrease in the crystallinity of HDPE. The mechanical restraint decreased the elongation and Izod impact strength. In the presence of coupling agent, maleic anhydride-grafted HDPE (HDPE-g-MAH), the tensile modulus and strength enhanced significantly because of enhanced interphase adhesion. However, the elongation and Izod impact strength decreased because of enhanced mechanical restraint on account of increased phase interactions. Scanning electron microscopy showed a degree of better dispersion of TWF particles because of enhanced phase adhesion in the presence of HDPE-g-MAH. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 1826-1834, 2009