The effects of various types of compatibilizers on the mechanical properties of high-density polyethylene/ wood flour composites were investigated. Functionalized polyolefins, including maleated polyethylenes, polypropylene, and styrene-ethylene/butylene-styrene copolymer, were incorporated to reduce the interfacial tension between the polyethylene matrix and wood filler. Of these, maleated linear low-density and high-density polyethylenes gave higher tensile and impact strengths for the composites, presumably because of their better compatibility with the high-density polyethylene matrix. Similar but less enhanced improvements in the mechanical properties, depending on the compatibilizer loading, were seen for a maleated styrene-ethylene/butylene-styrene triblock copolymer, whereas maleated polypropylene only slightly improved the tensile modulus and tensile strength, which increased with increasing compatibilizer loadings. Scanning electron microscopy was used to reveal the interfacial region and confirm these findings. Dynamic mechanical thermal measurements showed the interaction between the filler and the matrix. Fourier transform infrared spectroscopy was used to assign the chemical fixation and the various chemical species involved on the surfaces of the fillers before and after surface treatment.