Summary: The effect of silica containing immobilized nanosilver (SiO2 − Ag) on the structure, mechanical and bactericidal properties of wood-filled high density polyethylene composites compounded in a co-rotating twin-screw extruder and then injection molded was investigated. Maleated high density polyethylene (MHDPE) was used to improve mechanical properties, while spherical nanosilica containing immobilized nanosilver modified bactericidal properties of wood-filled HDPE composites. It was found that the addition of silica in the presence of MHDPE improves adhesion between HDPE and wood fiber. DMTA measurements confirmed these observations, showing toughening effect in the presence of SiO2 − Ag and MHDPE. The gradual enhancement in tensile and flexural strength, elongation and Charpy impact of the wood-filled HDPE composites containing MHDPE and SiO2 − Ag was observed. Nanosilica practically did not affect tensile and flexural moduli of the wood-filled HDPE composites, pointing to a toughing effect. Wood-filled HDPE composites modified with nanosilica containing immobilized nanosilver were found to be active against Escherichia coli, Staphylococcus aureus and Salmonella typhimurium whereas unmodified (without SiO2 − Ag) did not show this efficacy. Thus wood-filled HDPE composites containing SiO2 − Ag are effective bactericidal materials.