Hot cracking is one of the main problems in casting magnesium alloys and their joining with the use of welding. Despite a great body of research, which has led to the establishment of the hot cracking criteria and preparation of our models simulating the crystallisation of the casts and the crystallisation processes of the welds, in industrial practice such kind of cracking is still a significant problem. Conducted tests have enabled the determination of high-temperature brittleness range HTBR (High-temperature brittleness range) of AZ91E, understood as difference value of temperature between the temperature NST (Nil-strength temperature) and ductility recovery temperature DRT (ductility recovery temperature). It was stated that the widest range of HTBR appears in material in delivery state, due to biggest share of fusible inter-metallic phase Mg17Al12. Hot cracking in AZ91E alloy appears in areas of melted massive phase Mg17Al12 by disruption of the liquid film. Crack development follows along partially melted crystals of the solid solution Mg(a). High-temperature brittleness range HTBR determining the weldability of the AZ91E alloy depends on the heat treatment of this alloy. For alloy in delivery state it is 143°C, whereas after solution heat treatment it equals 94°C, and after solution heat treatment with ageing it is 91°C. Therefore, it is worth repairing the casts from AZ91E alloy in after-heat-treatment-condition.