The dynamic behaviors of W adatoms and clusters on W(111) were observed by field ion microscopy. The activation energy of single W adatom is higher than the activation energy of lots of W adatoms. That implies a sinlg W adatom is more stable. The occurrence temperatures of the dynamic behaviors like terrace diffusion, ascending, descending and detachment motion on W(111) are quite near. These experimental results explain the mechanism of single atom W tip.