The emission property of a Spindt-type platinum field emitter was greatly improved by operating in carbon monoxide ambient with appropriate operating parameters. After sufficient aging, the emitter was operated in carbon monoxide ambient up to 10−3 Pa, at the emission current of 1 μA. The emission current first decreased in accordance with the gas introduction, but turned to show rapid increase when the gas pressure was increased to 10−3 Pa. The current stability, as well as the operating voltage, was improved by this treatment. The apex of the emitter was examined with the Seppen–Katamuki analysis technique, in which detailed information on the emission area and effective work function can be read from the diagram plotted with the intercept and slope of a Fowler–Nordheim plot. The analysis suggested reduction of the effective work function is a major reason for the improvement of the emission property. © 2000 American Institute of Physics.