Using a field ion microscope, the diffusion behaviors and atomic processes of W atoms and clusters on W (1 1 1) surfaces were observed directly. The activation energy of W clusters diffusion on W (1 1 1) as a function of cluster size has an oscillatory and increasing behavior. But, the activation energy of a single W atom is especially high. The compact geometric structures are more stable and have higher activation energies of surface diffusion than structures with extra atoms at the periphery. Besides the terrace diffusion, other atomic processes such as the ascending, descending, detachment motion on W (1 1 1) surfaces were also observed. Unlike the general systems, their occurrence temperatures are quite near. These experimental results were used to discuss the formation mechanism of single atom W tips. (C) 2008 Elsevier B.V. All rights reserved.