This paper presents both the computational and experimental analysis of internal defects (shrinkage cavity and blow holes) to evaluate their effect on tensile properties of A356 casting alloys. For experimental testing, tensile test specimens with internal defects were produced by hanging an Al alloy wire whose melting point is slightly higher than the feeding temperature of A356 Alloy (700 °C ~ 710 °C). An alumina-silica ceramic fibre-based non-woven fabric (Ceramic Fibre Paper) was fixed on the wire to create artificial defect after the solidification of molten A356 alloy. Specimens with internal defects of various sizes were produced, and then tensile and impact tests were performed on the specimens. In addition to experimental investigation, computational analysis was conducted to determine the dependence of the alloy tensile properties on the internal defects present. A computational system for finite element analysis of casting components with internal defects was proposed. In this system, reverse engineering was first utilized to obtain the CAD models of internal defects. Then numerical calculations are performed to analyse the shapes of internal defects, reduce them to ellipsoids whose volume-sum approximately covers all points of the given defects, and then generate the final CAD model of the casting with internal defects. Finally, computational analysis using finite element method was performed on the defect-containing casting model. Results obtained by experiments and computer simulations were in agreement with some marginal differences. This agreement verified the reliability of proposed computational system for finite element analysis of casting components with internal defects.