N-doped TiO₂ (anatase) with high visible light photoactivity was obtained by the thermal treatment of nanotube titanic acid (denoted as NTA) in an NH₃ flow and investigated by means of X-ray diffraction (XRD), transmission electronic microscopy (TEM), diffuse reflectance spectra (DRS), X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR), and photoluminescence (PL). With increasing NH₃ treatment temperature at T = 400 to 600 °C, the anatase crystallinity of the N-NTA(400–600) samples was gradually enhanced, while at 700 °C a new phase, TiN, appeared in the N-NTA(700) sample. XPS results show that the doped N atoms incorporated into anatase TiO₂ exist in the form of NO. A revised explanation for the triplet ESR signals obtained from the N-NTA(500–700) samples was put forward, i.e. the g = 2.004 main peak is contributed by single-electron-trapped oxygen vacancies (denoted as V_o˙), while two weak peaks (g = 2.023, 1.987) are contributed by chemisorbed NO in well-crystallized anatase TiO₂. The visible light photoactivity is proportional to the height of the g = 2.004 main peak, which suggests that the photoactive centers are V_o˙-NO–Ti. The adsorbed NO molecule can effectively suppress the photoluminescence of V_o˙ defects, which facilitates photogenerated charge transfer to the surface reactive centers to conduct redox reactions. The higher the V_o˙-NO–Ti concentration, the better the visible light photoactivity. The highest photoactivity was obtained for the catalyst, NH₃-treated at 600 °C. But the formation of TiN at T = 700 °C can readily destruct V_o˙-NO–Ti photoactive centers, and thus readily decreases photoactivity efficiency.