Nanocrystalline TiO2 powders with different morphologies and grain sizes were successfully synthesized by the hydrothermal method. Different concentrations of hydrochloric acid (HCl), ammonium chloride (NH4Cl), ammonium sulfate [(NH4)2SO4], and ammonium carbonate [(NH4)2CO3] were used as additives in the hydrothermal process to investigate the effect of the concentration of ammonium (NH4+) and chloride ions (Cl−) on the phase compositions, morphologies, and grain sizes of the prepared TiO2. The as-synthesized samples were characterized by X-ray diffraction (XRD), transmission electron microscopy, Brunauer–Emmett–Teller analysis, and UV–Vis spectra. XRD results show that the as-synthesized powders are composed of anatase or a mixture of anatase and brookite. The grain size of the synthesized nano-TiO2 powder ranged from 5.0 to 11.3 nm, and the related BET specific surface area varied from 127.5 to 191.0 m2/g. The photocatalytic activities of the prepared TiO2 powders were evaluated by degradation of methylene blue (MB) in aqueous solution under UV light irradiation, and the results show that the photocatalytic performance of TiO2 powders synthesized with additives is improved compared with that of TiO2 prepared without any additives.