<p>In this study, the degradation of tensile properties due to casting defects in high-pressure die-cast A384.0 alloy was quantitatively evaluated from the perspective of the quality index, along with the influence of grain refinement on tensile performance. Tensile specimens were fabricated using a 250-ton cold-chamber high-pressure die-casting process, with mold cavities of 4 mm and 6 mm gauge diameters to induce differences in grain refinement. Representative casting defects affecting tensile properties included shrinkage voids and cold shuts. Cold shuts were observed in approximately 33–74% of the specimens, regardless of melt treatment or gauge size. The presence of cold shuts guided tensile crack propagation along the interface between abnormal and normal solidification regions, forming a characteristic facet surface on the fracture surface. The yield strength of A384.0 alloy was insensitive to the area fraction of casting defects, whereas tensile strength and elongation decreased significantly with increasing defect fraction. The defect susceptibility coefficients of the quality index for specimens containing only shrinkage voids were 0.87 and 1.88 for the 4 mm and 6 mm gauge sections, respectively, reflecting the effect of grain refinement. In contrast, when only cold shuts were present, the coefficients increased approximately threefold to 2.52 and 6.25, respectively. Furthermore, the maximum tensile strength achievable under defect-free conditions was markedly reduced under cold shut only conditions to 535 MPa and 442 MPa, compared with 717 MPa and 991 MPa for specimens containing only shrinkage voids.</p>