Anatase fluoride doped TiO2 sol (F-TiO2) catalyst was prepared by a modified sol-gel hydrothermal method, using tetra butyl titanate as a precursor. The influences of F doping, temperature of hydrothermal, values of medium pH on the morphology and crystallization were studied. The microstructure and morphology of sol sample were characterized by XRD, TEM, FTIR, UV-Vis-DRS, particle size distribution (PSD) and XPS. The results showed that F-TiO2 particles in sol were spherical and partly crystallized to anatase structure, and dispersed in the aqueous medium homogeneously and that the average particle size was ca. 10.5 nm calculated from XRD and TEM results. It was also found that the addition of fluorine could improve the crystallization and adsorption of particles significantly, the photocatalytic activity for decomposition of formaldehyde were enhanced remarkably with the doping of fluorine. Possible mechanism of anatase F-TiO2 formed under hydrothermal conditions was discussed.