Two types of A356 alloy castings in initial and modified with nanoparticles condition produced by gravitational casting were studied. Samples, as-cut from the castings, were subjected to light optical microscopy (LM), thermal analyses, Electron Backscattered Diffraction (EBSD) and Scanning Transmission Electron Microscopy (STEM) analyses. Results, obtained by EBSD, confirmed that there is grain refinement in samples from castings with added nanoparticles compared to the initial ones. STEM analysis shows agglomerates of nanoparticles in examined foils. Nanoparticles’ position in the microstructure confirms the hypothesis that they act as nucleating sites during the alloy solidification, which is the reason for observed fine-grained microstructure.