The mechanical properties of polyethylene (PE)/polypropylene (PP)styrene-ethylene/butylene-styrene (SEBS)/mica composites were investigated as a model composition. Four types of mica with different particle sizes and different aspect ratios were used. These composites were extruded by a twin-screw extruder and subsequently injection molded, and their mechanical properties, including their tensile, bending, and impact properties were evaluated. The addition of mica to PE/PP/SEBS effectively improved its mechanical properties; the addition of SEBS as a compatibilizer likewise enhanced the mechanical properties compared with those of PE/PP composites without SEBS. The use of larger average particle size and higher aspect ratio mica, such as M-35, M-27, and P-33, effectively increased the tensile and bending properties of the PF/PP/SEBS/mica composites. The morphologies of the composites with an added mica as observed by scanning electron microscopy, showed good dispersion between the matrix polymer and the mica.