The influence of the interfacial area and the particle size of CaCO3 filler particles on the mechanical and thermal properties of high-density polyethylene (HDPE) was studied in this work. The HDPE-based nano- and microcomposites were manufactured by using an industrial compounder system. The tensile, impact, creep, flexural and hardness properties of the filled and unfilled HDPE samples were investigated. The experiment revealed that the addition of both micro- and nanoparticles increased the tensile and flexural modulus of unfilled HDPE. However, it was observed that the addition of these particles did not have a significant effect on the tensile and flexural strength of unfilled HDPE. On the other hand, the presence of these particles decreased the elongation of break of unfilled HDPE. The impact strength of filled HDPE composites decreased slightly with both micro- and nanoparticle contents. The nanoparticle at high stress level (16 MPa) is more effective on the creep behavior of unfilled HDPE than on microparticles. However, microparticles were found to be more effective at low stress levels (8 and 12 MPa). It was found that the particle size has a profound effect on the thermal and physical properties of unfilled HDPE, such as density, melt flow index and vicat softening temperature. The results showed that the size of filler particles has a significant effect on the mechanical and thermal properties of the unfilled HDPE. Therefore, the size selection of constituent materials of nano- and microcomposites is an important consideration because it directly affects the functional performance of particle-filled HDPE nano- and microcomposites.