Face-centered cubic (fcc) Ru nanoparticles (NPs) have been recently synthesized by reducing Ru(III) acetylacetonate, despite bulk Ru having a hexagonal close-packed (hcp) structure. We observed the formation processes of fcc and hcp Ru NPs using X-ray diffraction and X-ray absorption fine structure. We concluded that the strong Ru-O bond between Ru and the acetylacetonate ligand leads to the formation of the fcc structure.