Gallium oxynitride (GaON) nanoparticles were synthesized through three steps; (i) hydrothermal treatment of an aqueous solution containing Ga(NO3)3, hexamethylenetetramine (HMT), and acetylene black, (ii) calcination, and (iii) nitridation. The presence of acetylene black in the hydrothermal treatment is effective for the synthesis of Ga2O3 nanoparticles after the calcination. The intermediate obtained after the hydrothermal reaction possessed no detectable Ga particles in the TEM observation, although the presence of Ga was confirmed in the EDS measurement. This means that acetylene black (AB), in this study, cannot play a simple role as a template. The GaON nanoparticles obtained from the Ga2O3 nanoparticles through nitridation possessed a higher oxygen content than that from Ga2O3 obtained by hydrothermal synthesis without acetylene black and the subsequent calcination. The obtained GaON nanoparticles show higher photocatalytic NOx decomposition activity than bulk GaON synthesized under the same conditions except without acetylene black in the hydrothermal reaction, because of the longer absorption edge and the higher specific surface area. In addition, the effect of nitridation temperature and time on the obtained GaON nanoparticles and their photocatalytic activity was also investigated. Consequently, nanoparticle morphology of a precursor for GaON is important not only for high surface area but also for high visible-light response.