Current study investigates the effect of addition of different amount (0 - 2 wt.%) of Silicon Nitride (Si3N4) nanoparticles in Al alloy (LM 6) on microstructural and mechanical characteristics. Al-Si3N4 nanocomposites are fabricated through ultrasonic assisted stir casting (USC) method. The fabricated samples are subjected to microstructural and mechanical characterization. Optical microscope (OM) is employed to conduct microstructural characterisation while Field effect scanning electron microscope (FESEM) is employed to check the dispersion of Si3N4 nanoparticles in Al matrix. FESEM images of nanocomposites confirm homogeneous dispersion of reinforcements. Energy dispersive X-ray Spectroscopy (EDAX) has been used for elemental analysis. Elemental mapping has also been performed on the specimens. Results of EDAX analysis and elemental mapping confirms the presence of Si3N4 nanoparticles in Al matrix. Grain size of all the developed samples is also measured. Grain size is decreased with increase in wt.% of Si3N4 nanoparticles. Al-2Si3N4 nanocomposite possesses 71.84% reduction in grain size compared to the grain size of base alloy. Density values of the developed samples are experimentally evaluated using Archimedes’ principle. Microhardness of base alloy and nanocomposites are also evaluated. Increased amount of reinforcement resulted in increased microhardness (~35%) and density values (~0.738%). Mechanical and microstructural characterization indicated good interfacial bonding between Al matrix and Si3N4 reinforcements.