A hydrothermal precursor was first obtained by isopropyl titanate reacting with tetramethylammonium hydroxide (TMAOH), which acts as a source of nitrogen and carbon. A facile post-thermal treatment was employed to enhance the crystallinity and visible light photocatalytic activity of the as-prepared precursor. The resulting products of post-thermal treatment between 200 °C and 700 °C display different colours from brown to white. Black N-doped TiO₂ nanoparticles modified with carbon (denoted as N-TiO₂/C) were obtained at 300 °C, while yellow N-doped TiO₂ nanoparticles (denoted as N-TiO₂) were obtained at 500 °C. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) were applied to characterize N-TiO₂/C, N-TiO₂ and the evolution process during thermal treatment. The results show that for both N-TiO₂/C and N-TiO₂, nitrogen was doped into the lattice, thus narrowing the band gap and increasing the absorption in the visible light region. Moreover, for N-TiO₂/C, the carbon species modified on the surface and between the nanocrystals enhanced the visible light harvesting and increased the adsorption of the dye in the photodegradation measurement. The photocatalytic performance under visible light irradiation is N-TiO₂/C > N-TiO₂ > undoped TiO₂.