Surface treated bottom ash was utilized as a filler to prepare plastic composites. Bottom ash surface was modified with octyltriethylammonium bromide (OTAB) or hexyltriethylammonium bromide (HTAB) by ion exchange procedure in methanol solution. Surface-modified bottom ash was then melt-compounded with HDPE to investigate its compatibility with the matrix material. The mechanical properties such as tensile, compressive, and impact strength, in addition to morphologies, were analyzed to determine the advantage of surface treatment compared to using bottom ash without treatment. Tensile strength and compressive strength increased with increasing bottom ash content.