The fractographs of Mg-9Gd-4Y-0.6Zr alloy specimens which were tensioned at different temperatures were investigated by optical and scanning electron microscopy, respectively. The results showed that different slip systems were activated at different temperatures, which were responsible for varied deformation mechanisms and fracture mechanisms. At 25 °C, the enabled slip systems were few and only the slip systems on basal plane were able to be activated, so cleavage fracture was observed. At −196°Cthe number of enabled slip systems increased, prismatic or pyramidal slip maybe occurred, so that an obvious low-temperature plasticity was observed, while fracture mechanisms were mainly microvoid coalescence fracture with cleavage fracture in local areas. At 250 °C, 300 °C and 350 °C, the multisystem slips on the basal planes, the prismatic planes and the pyramidal planes were able to be activated, while fracture mechanisms were also microviod coalescence fracture. At 400 °C, recrystallization happened, grain-boundary sliding in new fine recrystallized grains made the plastic deformation easy, showing coarse-grain superplasticity phenomenon, and an intergranular shear fracture took place.