Magnesium matrix composite reinforced with SiC nanoparticles (n-SiCp/AZ91D) was fabricated by squeeze casting. The microstructure and mechanical properties of the nanocomposite at as-cast state (F), solid solution state (T4) and artificial aging state (T6) were investigated by optical microscopy, X-ray diffraction, scanning electronic microscopy (SEM) and tensile tests. The results show that the solid solution treatment can dissolve the β-Mg17Al12 eutectic phase in the matrix of n-SiCp/AZ91D composites to produce single supersaturated solid solution α-Mg phase and the ultimate tensile strength and elongation are significantly improved which can be up to 265 MPa and 13.7%, respectively. After aging treatment, the ultimate tensile strength and yield strength of the nanocomposite are further improved, which is 275 MPa and 145 MPa, respectively. SEM results show that the β-Mg17Al12 phase is mainly precipitated in grain and at grain boundary as continuous or non continuous forms. In particular SiC nanoparticles have an important effect on the morphology, size and distribution of the β-Mg17Al12 precipitation phase and they result in the fine and uniformly dispersed secondary precipitate phase to be formed, which gives full play to the secondary precipitation strengthening effect. Finally, the fracture morphology of n-SiCp/AZ91D composites at different heat treatment conditions were observed by SEM and the fracture behavior were analyzed and discussed