High pure ultrafine AgO powder was synthesized by ozone oxidation of AgNO3. Silver valence change of AgO at different temperatures was determined by differential thermal analysis. AgO powder were also characterized by XRD, SEM and XPS to analyze the phase and morphology and the silver valence change of the powder heated at different temperatures. The results show that the crystal structure of the prepared ultrafine AgO powder belongs to monoclinic system and AgO particles are of virgulate shapes. The colour of ultrafine AgO powder changes obviously after heating. In the heating process, with increasing of the temperature, the content of AgO decreases gradually, the content of Ag2O increases firstly and then decreases and the content of Ag increases gradually. When the temperature exceeds 350 oC, AgO decompose completely, 46.49 % of which transforms into Ag2O and 53.51% transforms into Ag. And complete decomposition to Ag occurs at 400 oC. The XPS spectra of O1s on different powder surfaces are quite distinct, reflecting the lattice structure change of powder