The morphology of primary Al3(Sc, Zr) particles in as-cast microstructure of Al-Zn-Mg-Cu-Zr-Sc alloys and the grain refinement mechanism of Sc on as-cast microstructure of Al-Zn-Mg-Cu-Zr alloys were investigated by optical microscope, scanning electron microscope (SEM) and energy spectrum analysis. Primary Al3(Sc, Zr) particles precipitated from the melt is an effective nucleating agent of α(Al) matrix with metastable Al3Zr (L12) as nucleation. The particle is of layered construction. Sc and Zr in primary Al3(Sc, Zr) particles distributed interleavingly with the Zr-rich core. Adding 0.20wt%-0.60wt% Sc to Al-9.0Zn-2.5Mg-2.5Cu-0.15Zr alloys, coarse dendrite was changed into fine equiaxed grains. The higher content of Sc of alloys is, the finer as-cast grain is.