Abstract: The microstructural evolution of ZL101 semi-solid primary α-Al under ultrasonic treatment and isothermal treatment was investigated via ultrasonic stirring and isothermal process.On this basis, the relation with the conventional description method and the focus and the shortcomings were analyzed using box-counting dimension in describing α-Al morphology microstructure. The results show that the ultrasonic field has significant effect on the primary structure of ZL101, and the microstructure of 585 ℃ is optimal (equal-area-circle diameter is 131.45 μm, the shape factor is 1.479 and the solid fraction is 0.67); when the ZL101 semi-solid morphology is described on the fractal dimension, the box-counting dimension can be well integrated as a comprehensive expression of the solid-phase ratio and the shape factor, while the correlation degree with the equal circle diameter is minuteness.