Influence of oxygen on the field emission of singlewalled carbon nanotubes (SWCNTs) was studied using a fieldemission 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 anodecathode voltage (IV 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 IV curves.