Abstract. Dynamical change of structure of Rh supported on H-USY and Na-USY zeolite was measured by Quick XAFS technique operated under in situ conditions. In the atmosphere of H2, formation of the Rh clusters consist of 13 atoms was observed in H-USY and Na-USY. On the other hand, in the atmosphere of O₂, formation of the highly dispersed Rh oxide was observed in H-USY, whereas Rh oxide species did not disperse in Na-USY. In the reduction processes after heating samples in the atmosphere of O₂, the highly dispersed metal Rh particles in the H-USY agglomerated with increasing temperature above 553K. Whereas, the generation of stable Rh clusters were observed on the Na-USY. From these finding, it was conclude that the structure and acid sites of USY affected on the dispersion of Rh oxide particles and the generation of metal Rh clusters.