In this book, nanoparticle reinforced Al and Al alloy based nanocomposites have been prepared by reactive stir casting technique and the influence of microstructural features and parameters on the mechanical properties has been studied. Three different types of nanocomposites were prepared: (i) Al/(Al3Ti+Al2O3) (ii) Al-Cu/Al2O3 (iii) Al-Cu/(Al2O3+Al3Ti). Microstructural parameters such as screw and edge dislocation density were evaluated by using x-ray diffraction analysis. Nanomechanical properties were studied by using nanoindentation and nanoscratch test. The strength properties of the nanocomposites were measured by conducting tensile tests and the properties are correlated with the respective microstructural characteristics, which are observed by scanning electron microscope and transmission electron microscopes.