This study reports on ion irradiation-induced grain size variations in nanocrystalline SiC films on Si substrates. The SiC grains with average size ranging from similar to 2 to 20 nm were embedded in amorphous SiC matrices. Irradiation was performed using 5 MeV Xe23+ ions to 1.15 x 10(16) ions/cm(2) at 700 K. The irradiated films were characterized using X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. Significant grain growth is observed for smaller grains that tend to saturate at similar to 8 nm. In contrast, irradiation of larger grains (similar to 20 nm in size) leads to a decrease in the grain size, which could be associated with the production of lattice disorder within the grains. Homonuclear C-C bonds in the irradiated amorphous SiC matrix are found to be graphitized. This bonding transformation could limit or inhibit grain growth and contribute to the size saturation. The results from this study may suggest nanocrystalline SiC as a promising candidate structural or cladding material for applications in advanced nuclear reactors.