A method for predicting porosity defects in spheroidal graphite (SG) iron castings with sand mold using critical stress values specific to melt was studied by comparing experimental results with stress distribution obtained by numerical stress analyses including mold strength effects. The elastic perfectly plastic volume pixel finite element method (VOXEL-FEM) was introduced to calculate stress distribution of the castings. The analysis considered elastic deformation of mold and contacts between the mold and casting. Young's modulus of the mold was estimated from the relation between mold elasticity and strength. Porosity volume in the calculation was defined as the volume of element where average stress exceeds the critical stress. As a result of comparing the calculated results with porosity observations under various mold strengths, the critical stress was estimated to be about 3.5 MPa, confirming that defective regions can be quantitatively predicted with this critical stress. Furthermore, the proposed method was applied to a casting with green sand mold. The calculated results showed that green sand mold has little influence on stress distribution and porosity volume of the casting as shown by experimental results.