The field emission characteristics of a single micro-bundle of single-walled carbon nanotubes (SWCNTs) were investigated using field emission microscopy (FEM). Fowler-Nordheim plots revealed that the work function of the SWCNTs was reduced with increasing heating temperature, and reached a minimum value around 1000 °C, assuming that the β factor was constant during the heating process. Field emission patterns also demonstrated fine structures that were believed to be images of the cap of a SWCNT, which was in a clean state. The radius of the SWCNT micro-bundle was measured by transmission electron microscopy (TEM), and the β factor was calculated using two empirical formulae. Then, the work function of the SWCNT was determined from the slope, K, of its Fowler-Nordheim plot. The work function values were Φ1=4.76 eV and Φ2=4.88 eV, respectively.