Carbothermal reduction-nitridation (CRN) of SiO2-Al2O3-CaO powders was performed under various firing conditions to investigate the formation process of Ca-alpha sialon hollow balls composed of nanosized particles. Scanning electron microscopy and transmission electron microscopy observations of the samples obtained at different firing temperatures confirmed that solid spherical particles were formed at the early stage of the reaction, and nanosized particles were subsequently produced on the surface of these solid balls. From X-ray diffraction and energy-dispersive spectrometry analyses, it was found that the solid balls initially formed at 1450 degrees C were mainly amorphous and contained Si, Al, Ca, O, and a small amount of N. Further nitridation at 1450 degrees C gradually converted the solid balls into Ca-alpha sialon hollow balls over time. The results revealed that the formation of Ca-alpha sialon hollow balls depends on the formation of solid balls from the Si-Al-Ca-O liquid phase at the initial stage of the CRN process.