A gas field ion source (GFIS) has several advantages such as high current density, low energy spread, and high brightness. In this study, Helium gas was used in a GFIS, generated using the field ionization and field evaporation processes. The (110) oriented three adatoms tip (TAT) and single atom tip (SAT) were prepared with its ion emission stability 9.8% and 10.1% by the nitrogen field-assisted etching, respectively. The TAT and SAT were reproduced throughout the field evaporation and nitrogen field-assisted etching along the same crystal axis with its brightness 2.6 x 10(7) A/(m(2) sr) and 1.6 x 10(8) A/(m(2) sr), respectively. The field emission electron beam effective radii for three adatoms and single atom were calculated to be 2.90 angstrom and 1.47 angstrom, respectively, by Fowler-Nordheim plot, in which the single atom size was especially shown to be quite in good consistency with an atomic radius of tungsten.