TiO2 nanofibers with controllable diameters have been successfully fabricated by a facile electrospinning method. Their crystal structure and morphology features were monitored by X-ray diffraction and scanning electron microscopy. The photocatalytic activity of TiO2 nanofibers was evaluated by the photodecomposition of rhodamine B aqueous solution. It was found that the diameter of the nanofibers, controlled by changing the tetrabutyl titanate content in the precursor solution, plays an important role on the photocatalytic activity of TiO2 nanofibers. There is an optimal value of diameter on the photocatalytic activity of TiO2 nanofibers, which increases with the increasing of fiber diameter up to about 200 nm and then decreases. The mechanism may relate to the dynamics of electron/hole recombination on surface sites.