Controllable synthesis of nanomaterials with different morphologies can significantly affect their properties. Here we report that the morphology and facet orientation of TiO2 nanocrystals can be readily modulated via a hydrothermal method using a simple morphology-controlling agent of NH4F. The photocatalytic activity of resultant TiO2 has been evaluated by photodegradation of methyl orange. The results indicate that the introduction of NH4F can be used to modulate the mophology and, thereby, the photocatalytic activity of TiO2. The obtained TiO2 with high-energy facet, small size, and large surface area can exhibit an improved photocatalytic efficiency, which may be promising for real application.