Abstract: Alow-temperature hydrothermal method with room temperature ionic liquid(IL,1-ethyl-3-methyl-imidazole acetate) and water mixture as solvent was used to prepare nanocrystalline TiO2 powders.The crystal phase,morphology,and grain size of the products were characterized by using X-ray diffraction(XRD),transmission electron microscopy(TEM) and selected area electron diffraction(SAED) techniques.It was found that high crystalline anatase TiO2 nanoparticles could be obtained at specific ILconcentrations.When ILvolume fraction was 0.5,crystallization of TiO2 could be enhanced with increasing the amount of ILin reaction medium,and vice versa.Besides,grain size of TiO2 nanoparticles could be tuned in a certain extent by changing the ratio of water and IL.These results were thought to be related with the ionic strength,viscosity and micro-structure of IL-water solution.