Argon ion sputter etching was applied to SUS420J2 stainless steel, oxygen free copper (OFC) and pure Ni plate specimens that were placed on a SUS304 stainless steel or an OFC target disk at a sputter power of 150–400 W for 2–180 min. Cone-shaped protrusions with bottom diameters ranging from 2–20 µm were formed at various intervals on the specimens. The field emission measurements showed that the SUS420J2 specimen has the best emission properties among the specimens, i.e., the lowest voltage to extract emission current, the largest upper limit current density of about 2 mA/cm2, and the largest field enhancement factor of 810, attributed to its sharp protrusion tip, small vertical angle and relatively wide protrusion intervals.