In the absence of any doping and modification, the anatase-to-rutile phase transformation was inhibited at high temperatures giving rise to highly thermal stable and highly crystalline anatase TiO 2 fibers. The initial formation of the TiO 2 (B) phase is found tobekey in inhibiting this transformation.Theintermediate structure of the TiO 2 fiber comprises an inner anatase core with an outer TiO 2 (B) shell, which has a specific crystallographic orientation with respect to the anatase structure. During the calcination process from 300 to 800°C, both the TiO 2 (B) shell and the bulk anatase crystal structure was preserved. At temperatures of 800-900°C the TiO 2 (B)-to-anatase transformation was finished and a near-pure and thermally stable anatase fiber was obtained. This final product shows the same activity as a standard commercial photocatalyst Degussa P-25 when measured against unit mass, and 5 times the activity when measured with respect to the unit surface area. The anatase TiO 2 fibers presented here have considerable interest as practical photocatalysts for water purification, as they can be easily recycled without a decrease in their photocatalytic activity and can be prepared at large scale and at low cost.