High surface area and high crystallinity structure of TiO2 have been concentrated for the synergistic promotion of performance in the previous studies. As high temperature heat treatment can lead the opposite development of crystallinity and surface area, high surface area TiO2 with high crystallinity at a lower temperature has been focused. In the present work, crystallized and mesoporous TiO2 nanofibers (NFs) with a high surface area were obtained by a simple water steam treatment at the low temperature of 150 degrees C without any template agents. We demonstrated that water steam could promote not only the removal of organic component but also the crystallization and phase transformation of TiO2 NFs. The mechanisms of the low-temperature phase transformation and the formation of mesoporous of TiO2 NFs in water steam atmosphere have been analyzed. The change of the surface area and pore size distribution of the mesoporous structures with increasing temperatures were also studied. Furthermore, photocatalytic properties of the mesoporous TiO2 NFs were characterized by the photodegradation of methyl orange. It suggested that the mesoporous TiO2 NFs heat-treated at 550 degrees C exhibited the highest photocatalytic activity for the synergistic effect of the crystallinity and surface area. In the future, such a facile, scalable, cost-effective approach can be used for the fabrication of mesoporous TiO2 NFs.