Abstract: The addition and control of oxygen content in electrode gap plays a key role in improving the energy conversion efficiency and power density of thermionic converter (TIC). Therefore, the different methods to add oxygen, as well as their effects on the output performance of TIC were systematically investigated. The work function of the electrodes that co-adsorbed oxygen and cesium is lower than that only adsorbed cesium, thus the output power density and conversion efficiency would be improved. There are mainly two ways to supply oxygen to TIC. One is to add oxygen or oxides directly into the electrode gap in the form of vapor phase by using steam source. Another is to provide oxygen by releasing it from oxide or oxygen-sensitive metal of the collector. By the comparison, it can be concluded that the TIC with oxygen-sensitive metal collectors has better performance with high reliability and long service lifetime. Therefore, selecting or designing a special material for collectors is considered to be a development direction of oxygenated TIC.