The hydrogen promotion technique for obtaining improved imaging condition for helium field‐ion microscopy of cobalt is described. The image quality is satisfactory for viewing stacking faults, twin boundaries, and phase boundaries between hcp cobalt and fcc cobalt in atomic dimensions. The stacking faults are bunched in hcp cobalt, the individual faults being separated on the average by ten atomic layers of the basal plane. Most of the faults jog every 20–30 Å. Many, parallel, thin, fcc plates were found in an hcp matrix. No effect of grain size on the transformation from the fcc to the hcp structure was observed. The ratio of the quantities of the two phases was affected by a small amount of impurity, and cobalt wire from one source had an fcc structure throughout the accessible temperature range and could not be made to transform to the hcp structure by various heat treatments.