One of the biggest disadvantages of wood, as a potential reinforcement for thermoplastics, is its hydrophilicity. The aim of this study was to evaluate the effect of chemical modification of wood flour on mechanical, thermal, and fire properties of filled high-density polyethylene composites. For this purpose, aspen flour was modified with maleic, propionic, and succinic anhydrides. The modified wood flour and high-density polyethylene were compounded into pellets by single-screw extrusion, and test samples were prepared by injection molding. Tensile and flexural tests, impact testing, limiting oxygen index, TGA, and SEM analyses were conducted both for modified and un-modified samples. Based on the test results, chemical modification enhanced the properties of thermoplastic composites. Depending on the chemical concentrations, the flexural, tensile, and impact strengths of the modified flour filled HDPE composites were improved slightly, while the tensile and flexural moduli of the samples were increased significantly. The limiting oxygen index (LOI) levels of samples with modified wood flour were slightly improved, and succinic anhydride provided higher LOI levels compared to the samples with other anhydrides. This showed that the composites filled with chemically modified wood flour were slightly more thermally stable than control samples. It appears that wood flour modified with maleic, propionic, and succinic anhydrides can be successfully utilized as filler in polymer matrices.