The fluidized bed of ultrafine particle of photocatalyst is applied to treat nitrogen oxides (NOx) by photocatalytic oxidation. Three different TiO2 particles with primary particle diameters of 7, 20 and 200 nm are used as the bed material. The removal of NOx using a two-dimensional fluidized bed of ultrafine TiO2 particles is investigated. The fluidized bed of 7 nm TiO2 exhibits high removal efficiency of NOx because of its large specific surface area. It is found that the amount of NOx removal is proportional to the specific surface area. The fluidization of three kinds of photocatalysts shows the following characteristics. Agglomerates of 7 and 20 nm particles appear to be so hard that they are not destroyed during fluidization because of large adhesion forces of particles. In the case of 7 and 20 nm particle systems, the bed height is found to increase progressively with the increase in gas velocity, while the bed expansion is observed to level off for 200 nm particle system. The entrainment rate of 7 and 20 nm particle systems is found to be smaller than that of 200 nm particle system.