Tensile tests were conducted on several cast aluminum specimens with different degrees of porosity. The effects of non-uniform void size and shape distributions, including spherical and non-spherical types, on stress-strain behavior resulting from different initiation mechanisms were investigated. A micro-mechanics-based statistical approach was employed, and the heterogeneous microstructures could therefore be modeled during the deformation process. The predicted changes of the distributions of void size and void shape generally agreed with experimental results. Void spatial variation was also quantified, and its effects on the level of failure were analyzed. The void spatial variation facilitated development of inhomogeneous deformation, which results in failure.