(Al2O3+Al3Zr)P/Al composites were in situ synthesized from Al-Zr(CO3)2 system. In the process of in situ reaction, electromagnetic stirring (EMS) is employed to improve the conditions of reaction between reactants powder and melt. The results indicate that the Lorenz force improves the kinetic condition and accelerates the nucleation of endogenetic particulates. The optimized electromagnetic density is 0.025T. Scanning electronic microscope (SEM) observation demonstrates that the sizes of reinforced particulates are controlled at 1~2 μm. Moreover, the particulates are well distributed in Al matrix.