An investigation was made on the viscous flow and microstructure in hypereutectic Al–Si alloys rheocast with the rotation of a stirrer at a high speed in the range from 30 to 70 rev/s. The value of apparent viscosity in Al–Si alloys solidifying with the rotation of stirrer at a speed above 30 rev/s remained almost unchanged to a certain level except a rapid change at the initial stage of solidification. The average size of primary silicon was 66±19 μm in an Al-21.5%Si-3.5%Cu-0.3%Fe-0.1%Mg alloy ingot rheocast at 70 rev/s under the cooling condition at a rate of 0.6 K/s in the melt. The primary silicon size became finer by the modification of microstructure with an addition of 0.01% Na or 0.01 and 0.1% P in rheocast and conventionally cast hypereutectic Al–Si alloys. A constant level of apparent viscosity was also observed during the high-speed rheocasting of an Al-15.4%Si-4%Cu-1.1%Ni-1%Fe-0.3%Mn-0.2%Ti-0.1%Mg-0.02%Zn-0.01%Na alloy ingot. It was depressed with the increase in rotation speed of a stirrer from 30 to 50 rev/s. The primary silicon size of the alloy ingot was 101±27 μm at a rotation speed of 30 s⁻¹, 96±25 μm at 40 s⁻¹ and 93±25 μm at 50 s⁻¹.