It is difficult to evaluate the strength of material which has defects, because the strength differs variously by the stress concentration, the defects size and the temperature. In this study, the evaluation method for material with defects was studied. As an example, non-combustible magnesium alloys to which some silicon was added were applied. At first, observation of the fracture surfaces of specimens with a small hole was carried out. Although some dimple can be observed near the hole on the fracture surface of specimen tested in 250℃, no dimple was in room temperature. By the observation it was clarified that the fracture mode near the defect can be divided into two modes, i.e. brittle fracture and ductile fracture. In addition, by tensile tests of specimens with a notch, change of the fracture mode was also observed by degree in stress concentration. Understanding the two problems, i.e. degree in stress concentration of the defects in material and relationship between the defects size and strength are future tasks.