In order to evaluate a casting material which had a variable tensile strength, a strength evaluation method was proposed. Tensile tests were carried out on a specimen with a particular artificial defect as the fracture origin. The fracture origin size could be controlled using the proposed evaluation method. Therefore, the tensile test results, which had slight scatter, were obtained. However, the shape of the artificial defect was different from the casting defect, thus the effects of each defect on the tensile strength might be different. Both the critical stress intensity factors at the beginning of the unstable fracture were then compared, and it was clarified that the artificial defect and the casting defect had the same effect on the tensile strength. By this method, a non-combustible Mg alloy with added-Si, which contained large casting defects, could be evaluated. It was clarified that there was no difference of tensile strength between the noncombustible Mg alloys with 1% and 2% Si, and the relationship between the tensile strength and temperature was obtained.