Effects of addition of Pt to PdO/alpha-Al2O3 catalyst on the catalytic activity for methane combustion have been investigated. The size distributions and the shapes of supported particles in catalysts and their behavior at high temperature were measured. The results were discussed in relation with the heat resistance of supported particles. Supported particles were characterized by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). The addition of Pt to PdO/alpha-Al2O3 catalyst did not only bring about the increase of catalytic activity, but it was also effective for preventing the catalyst deactivation in methane combustion. These effects were attributed to the higher dispersion of the supported particles and the suppression of the particle growth. From the in situ observation of the PdO/alpha-Al2O3 catalyst, it was clarified that the supported particles migrated and then coalesced with each other like dewdrops on the surface of alpha-Al2O3 support at the temperature range from 843 to 863 K. The particle growth of supported particles proceeded through the migration and coalescence of crystallites. On the other hand, with the Pt-added catalyst, the migration and coalescence of supported particles were not observed. Its EDS analysis revealed that the every supported PdO particle contained Pt. The supported PdO particles containing Pt have smaller contact angles and are flatter than PdO particles free from Pt.