Abstract: The semi-solid ZL201 alloys cast with near-liquidus casting technology were reheated, and its microstructural evolution and α-grain growth mechanism were studied by means of theoretical calculation and actual observation. The results indicated that the microstructural evolution of the α-grain formation consists of three stages, i.e., eutectic structure melting, grain spheroidizing and theoretical liquid-solid equilibrium. The α-grains grow in form of coalescence in the first two stages in accordance to Ostwald model in the third stage. In addition, the microstructure evolves faster with increasing reheating temperature,and the spheroidization of α-grain depends on the isothermal holding time. It implies that the homogeneous, fine and near-spherical microstructure can be obtained via thixoforming at relatively higher reheating temperature within shorter holding time.