An investigation was carried out on the grain refinement of Al–Cu alloys containing 4, 10 and 24 mass%Cu by the vacuum superstircasting with high rotation speeds of a stirrer up to 23 rev/s applied during solidification under continuous furnace cooling. The size of primary solid particles in these alloys consistently decreases, the grain structure becomes fine and the crystal morphology changes from rosette type to spherical type with the increase in rotation speed from 13 to 23 rev/s. The minimum size of primary solid particles at 23 rev/s is 133±32 μm in Al-4%Cu, 124±44 μm in Al-10%Cu and 120±51 μm in Al-24%Cu alloy. It is thus expected that the mechanical grain refinement during solidification of the alloy can be established by high rotation of a stirrer inserted in the solid-liquid coexisting zone of the alloy, since the dendrites formed are destroyed into non-dendritic primary solid particles by shearing operation with the rotation of the stirrer. The coarsening of primary solid particles (grain growth) under fluid flow is theoretically evaluated on the basis of the experimental results to determine the mixing diffusivity.