The effects of semi-solid isothermal heat treatment temperature and holding time on microstructures evolution of extruded AZ91 magnesium alloy were studied. The microstructures of hot extrusion AZ91 magnesium alloy are characterized by streamline structure and small recrystallized α-Mg equiaxed grain. In the semi-solid isothermal treatment, the phase of low melting point begins to melt firstly, then permeates along the grain boundary, and finally the semi-solid structure which is liquid phase surrounded by solid-phase particles is formed. With the increase of isothermal temperature, melt and separation of the solid phases become faster. At the isothermal temperature of 560 oC, with the extension of isothermal holding time, the liquid phases increase obviously, solid phases separate and tend spheroid constantly. After holding for 20 min, the alloy reaches the solid-liquid equilibrium state, Ostwald ripening mechanism begins to be clear, and grains coarsening become the main mechanism. The better semi-solid processing technological parameters of extruded AZ91 alloy are 560 oC and 20~30 min