Research on metal matrix nanocomposites has risen in recent decades due to their unique properties such as high strength, hardness and service temperature. In the present work, cast A356-SiO 2 nanocomposites were fabricated by vortex and compocasting techniques. Nano SiO 2 reinforcements were added into the melt in two different forms either as raw SiO 2 particles or as Al1100-SiO 2 nanocomposite stripes produced by continual annealing and roll-bonding process and the effect of SiO 2 addition and casting method on microstructural and mechanical properties of the cast nanocomposites were investigated. The results showed that in the continual annealing and roll bonding stage, number of SiO 2 agglomerates decreased and mechanical bonding and wettability between most of SiO 2 nanoparticles and the Al1100 matrix were improved by increasing the number of cycles. The results of the casting stage for fabrication of the final composites revealed that the average dendrite length was decreased by additions of nano SiO 2 particles in the form of Al1100-SiO 2 nanocomposite stripes but did not change significantly when the reinforcement was added as raw powder. For samples cast from fully liquid state, the average dendrite length was decreased from 314µm in the monolithic sample to 237µm in the composite sample. Furthermore, the average secondary dendrite arm spacing in the fully liquid cast samples was increased from 17µm for monolithic sample to 28µm for composite sample. This increase is believed to be due to very low thermal conductivity of nano sized SiO 2 particles. For the semisolid cast samples, presence of nano SiO 2 particles caused the average equivalent circle diameter of primary ? Al particles to decrease from 150 to 101µm for samples with 0.1 solid fraction and from 180 to 114µm for samples with 0.2 solid fraction. For composite samples, the average equivalent circle diameter of the secondary ? Al particle was also decreased. It was 49 and 28µm for monolithic samples cast with 0.1 and 0.2 solid fraction and 37 and 30µm for composite samples cast with 0.1 and 0.2 solid fraction, respectively. Hardness in all the composite samples was higher than those of the corresponding monolithic samples. The highest increase was observed in the semisolid sample fabricated by addition of nanocomposite stripes and cast with 0.2 solid fraction. The ultimate shear strength and shear yield strength of the composites produced by addition of nanocomposite stripes were significantly increased compared with those of the matrix alloy. In addition, the semisolid sample cast with 0.2 solid fraction retained about 45% of its ultimate shear strength and 55% of its shear yield strength at 300 ? C. These values for the monolithic matrix alloy was about 34% and 48%, respectively. Keywords : A356-SiO 2 nanocomposites, vortex method, compocasting, continual annealing and roll bonding process, microstructure, shear strength