In this paper, we first reported the crystalline transition from H2Ti3O7 nanotubes to anatase TiO2 nanocrystallines under low-temperature hydrothermal conditions (similar to 140 degrees C). A newly proposed mechanism for the crystalline transition from H2Ti3O7 nanotubes to anatase TiO2 nanocrystallines under low-temperature conditions is discussed in detail, which is supported by X-ray diffraction, high-resolution transmission electronic microscope, selected-area electron diffraction, and crystal structure models.