Two-phase aluminum-silicon-based alloys are widely used for premium quality castings for aerospace and automotive applications. While it is clear that silicon improves fluidity in the molten state, providing excellent castability to the alloy, and increases the tensile strength of the alloy, much needs tobe done to improve the understanding of the structure-property relationships in castings. This paper deals with the application of the finite element method and the OOF program to study the effect of size and shape of silicon particles on the stresses in the silicon particles and the aluminum matrix. The highest stress in the matrix increases with increasing particle size. Therefore, the yield strength of the microstructure containing coarse particles would be lower than the ones containing fine particles. Once the silicon particles with large aspect ratios crack of the microstructure containing large silicon particles yield, the effective stiffness of the aluminum atrix decreases which significantly increases the average stress in the silicon particles and the highest stresses in both the silicon particles and the aluminum matrix. This indicates that once the matrix yields, the potential for particle cracking increases dramatically.