Herein, we successfully fabricated in situ α-Al2O3p/Al-12Si composites by direct melt reaction method from Al-SiO2 system with aid of electromagnetic stirring. Through thermodynamics analysis, it was demonstrated that the aluminum (Al) melt and SiO2 powders were firstly reacted at 755.8 °C to form metastable γ-Al2O3 and then converted to stable α-Al2O3 at 1045.6 °C. Microstructure observations showed that electromagnetic stirring was effective to suppress the α-Al2O3 particle agglomerations in 5 vol% α-Al2O3p/Al-12Si composite, and the α-Al grains in the Al-12Si matrix alloy were greatly refined by the well dispersed in situ α-Al2O3 particles. The ultimate tensile strength (UTS), Brinell hardness and elongation of the T6-treated 5 vol% α-Al2O3p/Al-12Si composite were 325 MPa, 145 HB and 2.5%, respectively. Compared with those of the Al-12Si alloy, the UTS and Brinell hardness increased by 10.9% and 16%, while elongation decreased by 36.0%. Furthormore, it was found that the high temperature UTS of the above composite was higher than that of the Al-12Si matrix alloy at the temperature ranging from 25 to 350 °C, and the contribution rate (K) of the in situ α-Al2O3 particles to the UTS reached its maximum value (34.3%) at 300 °C. Taken together, the in situ 5 vol% α-Al2O3p/Al-12Si composite with high strength at both room temperature and high temperature is very promising as the piston material.