Abstract: TiO_2 nanocrystallite was synthesized by gas hydrolysis of titanium tetrachloride conducted in co-flow diffusion flames.TiCl_4 feed rate and O_2 mole fraction in the center tube were varied to tune the grain size and the anatase content of TiO_2 nanocrystallite.Transmission electron microscopy(TEM) and X-ray powder diffraction(XRD) results show that the nanocrystallite is spherical in shape,uniformly distributed within the narrow size range. The photocatalyst activity of TiO_2 nanocrystallites under UV(irradiation) is evaluated using RhB as a mode organic compound.The results indicate that when the anatase content of TiO_2 nanocrystallite is constant,the grain size is in the range of 9.4 nm to 15.5 nm,smaller size has higher photocatalytic activity;when the grain size is constant,with the increasing of the anatase content,the photocatalytic activity increases first and then decreases.The anatase content of 0.80 has the highest photocatalytic activity for RhB degradation.