The study is about the phenomena of ultrathin film covered metal tip surfaces by field ion microscope. Anneal the cobalt tip to 800 K, and the surface did not facet but become more complete or broken. It is for faceting phenomena of platinum covered cobalt tip which will increase the surface energy anisotropy, lower the temperature of faceting, which let (0001) and (1013) faces expand and result in the formation of crest lines at 600K. Because cobalt is a non-refractory material, the crest lines are not complete and pyramid structure is difficult to form.