Magnesium (AZ91D) matrix nanocomposite reinforced with nano-sized SiC particles (n-SiCp) was prepared by mechanical stirring and high intensity ultrasonic dispersion assisted casting. The tensile properties and fracture behavior of the magnesium alloy and its nanocomposites at elevated temperature were investigated. The results show that the addition of n-SiCp to AZ91D can improve the tensile strength of the nanocomposites, especially at high temperature. Furthermore the significant improvement of the elongation of the nanocomposites at high temperature is achieved, which results in the quite good ductility at elevated temperature. The fracture analysis on the nanocomposites shows that the fracture mechanism of the nanocomposites changes gradually from the brittle fracture at room temperature to the typical tough fracture at high temperature due to the addition of n-SiCp.