Plastics are rarely used in their pure form and are often modified by adding mineral fillers to improve their properties or reduce costs. This study examines the effect of calcium carbonate (CaCO₃) and expanded perlite on high-density polyethylene (HDPE). Formulations containing 5 to 20% by volume were prepared using a Brabender plastograph (220°C, 30 rpm). The results show that viscosity increases but remains compatible with processing. The modulus of elasticity improves significantly with CaCO₃, while perlite induces a more moderate improvement. Elongation at break increases for 5 and 10% CaCO₃, but decreases with perlite, reflecting embrittlement. The stress at break decreases beyond 10% for both fillers. DSC analysis reveals a decrease in melting temperature, slight with CaCO₃ but more pronounced with perlite. Finally, SEM confirms good dispersion and, for HDPE/Perlite, brittle-ductile behaviour.