An investigation was made for the improvement of microstructures and mechanical properties at an elevated temperature in Al–Cu alloys containing 10, 24, 30 mass% copper, Al-8 mass% Si and Al-4 mass% Ni alloys rheocast with the rotation of a stirrer at a high speed in the range from 20 to 67 rev/s. It was found that the relation among the size, d, of primary solid particles formed by fragmentation of dendritic crystals, copper content, C₀, and rotation speed of stirrer can be expressed by the following empirical equation:d=K(33−C₀)ⁿ for n=0.32, where K is a constant determined by the stirrer speed. The apparent viscosity of Al–Cu alloys was evaluated during the solidification with high-speed rotation of stirrer. Rheocast microstructures in Al-8%Si and Al-4%Ni alloys are compared with those in Al–Cu alloys on the basis of microscopic observations. The maximum stress and total elongation of the Al-30% alloy ingot rheocast at a rotation speed of 33 s⁻¹ are 24.0–28.5 MPa and 71%, repectively, while those of the Al-24% alloys are 19.7–21.2 MPa and 63–86s at 33 s⁻¹ and 21.2–22.7 MPa and 79–91% at 67 s⁻¹, respectively.