Catalytic activity of ZrO2 supported PdO catalysts for methane combustion has been investigated in comparison with Al2O3 supported PdO catalysts. It was found that the drop of catalytic activity owing to decomposition of PdO at a high temperature region (600-900 degrees C) was suppressed by using ZrO2 support. Temperature-programmed reduction (TPR) measurements of the catalyst with hydrogen revealed that the PdO of PdO/Al2O3 catalyst was reduced at the temperature less than 100 degrees C, whereas in PdO/ZrO2 catalyst the consumption of hydrogen was also observed at 200-300 degrees C. This result indicates that the stable PdO species were present in the PdO/ZrO2 catalyst. In order to confirm the formation of the solid solution of PdO and ZrO2, X-ray diffraction (XRD) analyses of the mixtures of ZrO2 and PdO calcined at 700-900 degrees C in air were carried out. The lattice volume of ZrO2 in the mixture was larger than that of ZrO2. Furthermore, the Pd thin him on ZrO2 substrate was prepared as a model catalyst and the depth profile of the elements in the Pd thin film was measured by Auger electron spectroscopy (AES). It was confirmed that Zr and O as well as Pd were present in the Pd thin him heated at 900 degrees C in air. It was considered that the PdO on ZrO2 support might be stabilized by the formation of the solid solution of PdO and ZrO2.