In the current research, nitrogen doped titanium dioxide as a visible-light active photocatalyst material has been synthesized through sol-gel method. The main factors affecting the photocatalytic efficiency of N-doped titanium dioxide nanoparticles, including nanoparticle size and percent of crystallinity, have been investigated using Taguchi robust design. Water to titanium precursor ratio (H+/Ti4+), nitrogen to titanium source ratio (TEA/Ti4+), pH, and temperature were the considered factors. To clarify the crystallinity, size distribution and morphology of the synthesized nanoparticles, XRD, PSA and SEM analyses have been conducted. The results of XRD analysis revealed that acidic pH improves the formation of crystalline phase. Also, the water to titanium content ratio is inversely correlated to the crystallinity. In the case of using triethylamine (TEA) as a doping agent, the formation of crystalline phase is promoted. Moreover, a stable solution forms around the acidic pH. Addition of water increases its stability, but addition of TEA decreases it. In nonacidic pH the size of nanoparticles becomes smaller and increasing the water to titanium content ratio causes to decrease the particle size, addition of TEA also has the same influence. The microstructure and surface morphology of the N-doped TiO2 nanoparticles shows a uniform geometry and size distribution. (C) 2017 Elsdvier B.V. All rights reserved.