The effect of the silicone oil content on the tensile properties and micromechanical deformation of high-density polyethylene/Al(OH)(3) composites was studied. With an increase in the silicone oil content, the elongation at break and notched Izod impact strength of the composites increased, and the local debonding deformation gradually transformed into homogeneous debonding deformation. A model characterizing the micromechanical deformation process was examined. The increase in the silicone oil content decreased the interaction between the filler particles and the high-density polyethylene matrix and increased the extension speed of the debonded region, which led to the changes in the tensile properties and micromechanical deformation. A good explanation for the tensile properties and micromechanical deformation was obtained through the model.