Based on the numerical simulation of solidification of castings, a thermal stress formula and a thermal crack initiation criterion are proposed. Using these formulas, cast steel wheels with a diameter of 800 mm and aluminum alloy electromagnetic casting (EMC) slabs with a size of 1300 mm × 480 mm are employed to testify the positions of cracks through conventional thermal elastic-plastic analyses and low magnifying structure observations. The results show that the numerical prediction of cracks agrees with the measured results, and the cracks do not necessarily occur on the defects such as shrinkage holes (wheel) and porosity (EMC slab). It is also found that surface temperature control is an effective means to avoid the crack formation.