The nano-sized TiO2 particles by the diffusion flame reactor were prepared and the effects of several process variables on the properties of TiO2 particles were investigated. The photo-degradation of phenol was also analyzed with the prepared TiO2 particles in the slurry type photocatalytic reactor. As the fuel/O2 input ratio or inlet TiCl4 concentration increases or as the total gas flow rate decreases, the TiO2 particles size increases. The specific surface area of TiO2 particles decreases as the inlet TiCl4 concentration increases. As the TiO2 particle size decreases or as the O2 flow rate increases, the photo-degradation efficiency of phenol increases by the faster photo-degradation rate. The larger the amount of anatase phase in TiO2 particles, the higher the photo-degradation efficiency of phenol is.