Influence of oxygen on the field emission of singlewalled carbon nanotubes (SWCNTs) was studied using a fieldemission microscope (FEM) and a quadrupole mass spectrometer. After heat treatment at 1000℃, the field emission of clean SWCNTs occurred. Then oxygen was introduced into the vacuum system and the curves of field emission current versus anodecathode voltage (IV curves) were acquired after oxygen adsorption and desorption, respectively. We have observed that oxygen adsorption on SWCNTs leads to a decrease in field emission current, indicating an increase in the work function. Oxidation etching occurred when the SWCNTs were annealed at about 1000℃ under a pressure of 10-4Pa, and the corresponding changes were observed in both field emission images and IV curves.