Abstract: The mechanical properties of as-extruded AZ80 magnesium alloy at temperatures of 450–525°C and strain rates of 3.0 s⁻¹ and 0.15 s⁻¹ were investigated by tensile tests. Zero ductility of alloy appeared at 500°C with a strain rate of 0.15 s⁻¹, while the zero strength and zero ductility of the alloy were obtained nearly simultaneously at 525°C with a strain rate of 3.0 s⁻¹. The results indicated that the lower strain rate accelerated the arrival of zero ductility. As the temperature increased, the failure mode of the alloy developed from trans-granular fracture to cleavage fracture and then to inter-granular fracture with the feature of sugar-like grains and fusion traces. The existence of the low-melting composite of β-Mg₁₇Al₁₂ and Al₈Mn₅ particles segregated near the Mg₁₇Al₁₂ phase along grain boundaries were demonstrated to be the reason for the brittle fracturing of the AZ80 alloy at high temperatures. Furthermore, microstructural evolution at temperatures approaching the solidus temperature was discussed to clarify magnesium alloy’s high temperature deformation mechanism.