Abstract: Tensile tests were conducted on high strength and heat resisting GW123K magnesium alloy at room temperature with strain rate of 10-5,10-3,1 s-1,respectively. Then the microstructure and fracture surface were observed with optical microscopy (OM) and scanning electron microscopy (SEM) to study the effect of strain rate on the ductility of the magnesium alloy. Results show that the ductility of the magnesium alloy improved with the increase of strain rate. When the magnesium alloy fractured, the strain was 0.13, 0.12, 0.25, respectively, which showed abnormal strain rate sensitivity of the alloy.