The optical microscope, SEM and EDS were adopted to analyze the rheoforming solidification morpholo-gies and microstructures of deformed AZ91D magnesium alloy after isothermal treatment in semisolid state. The re-sults show that primary α phase can be formed through attachment growth, when the liquid fraction is small; and primary α phase will grow in dendrites, when the liquid fraction is high. Eutectic solidification is carried out in the ways of both dissociated growth and symbiotic growth, de-pending on the morphology of primary α phase. Liquid with eutectic concentration solidifies into lamina eutectic. During solidification of the liquid pools inside grains, α phase can be formed through attachment growth. The eutectic solidifica-tion was mainly carried out in the dissociated pattern. The optical microscope, SEM and EDS were adopted to analyze the rheoforming solidification morpholo-gies and microstructures of deformed AZ91D magnesium alloy after isothermal treatment in semisolid state. The re-sults show that primary a phase can be formed through attachment growth, when the liquid fraction is small; and primary α phase will grow in dendrites, when the liquid fraction is high. Eutectic solidification is carried out in the ways of both dissociated growth and symbiotic growth, de-pending on the morphology of primary α phase. Liquid with eutectic During solidification of the liquid pools in grains, α phase can be formed through attachment growth. The eutectic solidifica- tion was mainly carried out in the dissociated pattern.