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The mechanical properties and deformation mechanism of semi-continuously casting and as-extruded AZ70 magnesium alloys in a wide range of grain sizes (from 14 to 103 μm) were investigated at 653 K and 1×10-3 s-1. It is discovered that with reducing grain size, flow stress is weakened and plasticity is improved and even superplasticity exhibits. SEM and OM were used to clarify the deformation mechanism. It is suggested that dynamic recrystallization (DRX) is the coordination deformation mechanism of grain boundary sliding (GBS) for coarse grain, and cavity and intracrystalline slip are the coordination deformation mechanisms of GBS for fine grain.