We evaluated the strength of a cast non-combustible magnesium alloy that is intended for use in automobile engines. This alloy contains thin sheet-like oxides because of its production process. The size of these thin sheet-like oxides varies, and this means that the temperature dependency of the strength characteristics of the alloy cannot be determined by a tensile test alone. Initially, specimens of the alloy having an artificial pre-crack of 5 [mm] to simulate the sheet-like oxides were subject to tensile testing. The testing was carried out at room temperature (23 [°C]), 150 [°C], 200 [°C] and 250 [°C]. 250 [°C] is the temperature of the environment in which the alloy will be used in automobile engines. All the fractures propagated from the artificial pre-crack and the alloy exhibited its maximum strength at 150 [°C]. To investigate the temperature dependency of the fracture mechanism, a scanning electron microscope (SEM) was used to observe the region near the tip of the pre-crack on the fracture surface of the specimens tested at room temperature and at 250 [°C]. On the fracture surface of the specimen tested at 250 [°C], some dimples were observed over some of the surface near the tip of the artificial pre-crack; however, the fracture surface of the specimen tested at room temperature showed brittle fracture surface morphology over the entire area. For the magnesium alloy, the critical resolved shear stress for non-basal slip, which is a characteristic of a hexagonal close-packed metal, is dependent on temperature. Therefore, ductile to brittle transition was considered to be occurred at a temperature between room temperature and 250 [°C]. Mechanical restraint is also considered to be affected the ductile to brittle transition.