Microstructure evolution and the quantitative relationship between theα-Mg grain size as well as shape factor and process parameters of both the semi-continuous cast and extruded AZ80 magnesium alloy were investigated after semi-solid isothermal treatment（SST）.The results show that

with a given parameters

theα-Mg grain size exhibits more spherical with more uniform distribution in the extruded AZ80 magnesium alloy after SST

and the liquid phase in theα-Mg grain is generated from intracrystalline partial melting priority due to solution elements enrichment and high energy reserves

while the semi-continuous casting AZ80 magnesium alloy

the liquid phase in theα-Mg grain is caused by liquid phase entering inside the grain due to the dendrite arms coalescing.After SST with different parameters for extruded AZ80 magnesium alloy

theα-Mg grain sizes follows Gauss distribution.At 570℃for 20 min

the distribution of solid and liquid phase is relatively homogeneous

and the shape factor ofα-Mg grain size mainly concentrates in high coefficient area with relatively high average shape factor value of0.86.