The Mg-4Zn-0.5Ca alloys extruded at 280°C by different ultra-slow extrusion speed (0.01, 0.05, 0.1 and 0.5mms−1) were investigated in present study. The results show that both the volume fraction and average size of DRXed grains increase with the increase of extrusion speed, which is different from that depicted by present Z parameter. To fit the extrusion conditions with low temperature and ultra-slow extrusion speed, a more accurate estimate/measurement of the actual temperature in the expression of the Z parameter is needed in the further study. Even though the amount of precipitates decrease with the increasing extrusion speed, the increasing amount of DRX regions lead to the decreased intensity of basal plane texture. Superior mechanical properties with the yield strength of 324.5MPa, ultimate tensile strength of 371.1MPa and elongation to failure of 7.9% are achieved in present Mg-4Zn-0.5Ca alloy extruded at the speed of 0.01mms−1. As the extrusion speed increased from 0.01 to 0.5mms−1, the strength decreases which is accompanied with the increase of elongation.