The effects of Rh support materials and their loading amount on catalyst performance were investigated. As a result, Zirconium-based oxide (ZrO₂) showed better capability as a Rh support than alumina (Al₂O₃). Another finding was that the Rh dispersion rate in the washcoat is a key factor affecting the catalyst performance. The reasons for the superiority of ZrO₂ to Al₂O₃ are deemed as follows: 1) Even after severe thermal aging, ZrO2 can retain single-nano-size Rh particles by the SMSI. 2) On the ZrO2 support, Rh is more probable to retain metal state than on the Al₂O₃ support.