A simple sol-gel process was developed to synthesize a nitrogen-doped titanium oxide (TiON) nanoparticle photocatalyst. Our work shows that the precursor ratio (tetramethyl ammonium hydroxide (TMA)/titanium tetraisopropoxide (TTIP)) has significant effects on the structure, composition, and optical properties of TiON nanoparticles. At high initial TMA/TTIP ratios, crystallization of the sol-gel TiON nanoparticle powders was sluggish, requiring a higher calcination temperature. As a high calcination temperature caused greater nitrogen loss, a high initial TMA/TTIP ratio in the TiON precursor did not ensure a high N/Ti atomic ratio in the TiON powders. By optimizing the initial TMA/TTIP ratio and the calcination temperature, crystalline powders were obtained with high nitrogen concentrations. These sol-gel TiON nanoparticles showed strong visible light absorbance.