We have investigated the bonding nature between oxygen and sodium on Si(113) surfaces using photoemission spectroscopy. It was found that the modified Auger parameter decreased drastically from about 2065.4 to 2062.4 eV upon oxidation of 1 monolayer Na/Si(113) surface at room temperature. The modified Auger parameter for O-2/Na/Si(113) surface at room temperature was comparable to that for the typical sodium oxides, such as Na2O2 and Na2O, having ionic bonding character between oxygen and sodium. These results suggest that the bonding nature between oxygen and sodium on Si(113) surface at room temperature be ionic. O 1s spectra taken from O-2/Na/Si(113) surface showed two oxygen components, which means that there are two bonding sites of oxygen at room temperature. The lower binding energy component of O 1s core level peaks, depended on sodium coverage at room temperature, was observed at binding energy between those of Na2O2 and Na2O, and decreased with increasing sodium coverage at low temperature. The concentration ratio [rho(Na)/rho(O bonded to Na)] is about 1 from simple calculation. These results suggest that the lower binding energy component comes from oxygen being at the interface between sodium and silicon substrate, forming Si-O-Na bridge configuration.