Friction stir welding (FSW) was performed in the current research on Al 5059 alloy. Nano-sized SiC (n-SiC) particles were incorporated during FSW for improving mechanical characteristics of welds. Effect of tool rotating speed and multiple FSW passes was studied on SiC dispersion, micro structural changes and mechanical characteristics of composite joints. Results reflected that rotational speed and multiple FSW passes have significant impact over the distribution of SiC reinforcement. Further, the pattern of SiC distribution in the base metal strongly affects the micro structural evolution and mechanical characteristics of welds. An increase of ~55% in microhardness values was achieved in the stir zone of composite joint. Correspondingly, higher ultimate tensile strength (342 MPa) value was obtained as compared to base metal (321 MPa). The increase in mechanical characteristics is mainly owing to grain refinement caused by recrystallization and pinning effect of SiC particles. Micro structural study was carried out using metallurgical microscope and dispersion of n-SiC was observed using TEM. The results of mechanical testing of the present study are in strong corroboration with the micro structural analysis.