Abstract: XPS studies show that the dominant surface oxide is Nb_2O_5 for air exposed Nb metal at both room temperature and 300 ℃. The thickness of the room temperature Nb_2O_5 formed by 18 hours air exposure is less than the photoelectron escape depth. The room temperature Nb_2O_5 can be converted into Nb metal after heating the sample in vacuum at 350 ℃ for 30 minutes. If the surface Nb_2O_5 is formed at above 300 ℃, however, it can not be converted into Nb metal after heating the sample in vacuum at 350 ℃ for as long as 4 hours. It is suggested that the very thin room temperature surface Nb_2O_5 can differ from the bulk Nb_2O_5 on thermodynamical property.