A preliminary examination to understand a lowering mechanism of workfunction on theZrO/W(100)surface is reported. X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) were employed for this purpose. After deposition of about 3 monolayers of zirconium onto clean W(100) surface followed by heating in an oxygen ambient, the zirconium overlayer completely changed to ZrO 2 layer. In depth profile of this surface, the ZrO 2 was detected deep in the bulk, implying that the ZrO 2 was diffused into the bulk by heating in the oxygen. The surface that was covered with the ZrO 2 showed an unclear p(1 × 1)pattern. Following flash heating in an UHV gave rise to disappearance of the ZrO 2 and formation of Zr-O complex layer, which was accompanied by lowering of workfunction. A p(4 × 2)superstructure pattern was observed in the LEED observation on the surface. Depth profile on the surface showed that the zirconium exists at the utmost surface, taking the form of Zr-O. These results imply that theZrO/W(100)surface is formed by transformation from the ZrO 2 to the Zr-O complex and the taking the particular arrangement of Zr-O on the W(100) surface.