This study evaluates the effect of Calcium carbonate (CaCO₃) on the mechanical properties and structure of SEBS-compatible PA6/ABS composites. Composites consisting of 50/50 PA6/ABS, 5% SEBS, and 0–20% by weight CaCO₃ were fabricated by injection molding. Tensile and flexural strengths were determined according to ISO 527-2:2012 and ISO 178:2019, respectively. The tensile strength increased with filler content, peaking at 15 wt% CaCO₃ (24.72 MPa), while flexural strength reached a maximum at 10 wt% (32.51 N/mm2). FESEM revealed a uniform dispersion of CaCO₃ particles and strong interfacial adhesion at optimal filler contents, whereas agglomeration and microvoids occurred at higher loadings. The results demonstrate that moderate CaCO₃ addition enhances stiffness and strength through effective stress transfer, while excessive loading induces brittleness due to poor interfacial bonding. This study contributes to the optimization of hybrid polymer composites for structural, automotive and precision engineering