Using Field Emission Microscopy (FEM), the electron field emission from a bundle of single-walled carbon nanotubes (SWCNT's) was studied under vacuum better than 4.0×10-7Pa after being thermo-desorbed at different temperatures.Work function of the SWCNT's decreases as the temperature of desorption goes up.After heat treatment at 1000℃, the SWCNT's exhibits the lowest work function and their electron field emission pattern also shows a fine structure.As the temperature for thermo-desorption is raised further, the SWCNT's field emission power deteriorates and finally collapse of SWCNT's occurs.