The effects of the melt temperature on the microstructures and properties of in-situ Al-18%Mg₂Si composites were investigated. The results show that the coarse dendrite primary Mg₂Si phases transformed to the polygons and lumps and some transformed to small particles with the rising of the melt temperature. The primary Mg₂Si is the smallest with the average grain size of 7μm when the melt temperature is 870℃.The grain size of the primary Mg₂Si increased slightly when the temperature is over 870℃.The lamellar eutectic Mg₂Si transformed to small particles, then changed into the mixed microstructure with small and rodlike particles with increasing the melt temperature. The size of the eutectic group decreased first and then increased. The tensile strength, the elongation rate and the hardness increased first and then decreased with the rising of the melt temperature. The mechanical properties achieved the best value when the temperature is 870℃.The result and analysis of DTA show that the microstructure and mechanical properties of in-situ Al-18%Mg₂Si composites are the best, which results from the comprehensive function of multiple factors including large nucleation undercooling and the change of growth pattern.