The Polyformaldehyde (POM)/ethylene-vinyl acetate copolymer (EVA)/high-density polyethylene (HDPE) blends and POM/EVA/HDPE composites filled with nanometer calcium carbonate (nano-CaCO3) were prepared using a twin-screw extruder. The effects of the HDPE weight fraction on the flexural and impact properties of the blends and composites were investigated at room temperature. The results showed the flexural modulus and strength of the blends and composites decreased slightly with increasing the HDPE weight fraction. The values of the flexural modulus of the composites were higher than those of the blends, while the values of the impact fracture strength of the blends were higher than those of the composites under the same HDPE weight fraction. This indicated that the impact fracture toughness of the POM could be improved with the HDPE, while the flexural stiffness of the POM blends could be enhanced with the nano-CaCO3 to some extent.