Ultra high vacuum- field ion microscopy was used to study the atomic dynamics at W (111) surfaces. Over hundreds of the observations for atomic processes such as the terrace diffusion, ascending, descending, detachment motion on W (111) surfaces, we find that the activation energy of W clusters diffusion on W (111) as a function of cluster size has an oscillatory and increasing behavior. But, the activation energy of a single W atom is high anomalously. The compact geometric structures are more stable and have higher activation energies of surface diffusion than structures with extra atoms at the periphery. Unlike the general systems, the occurrence temperatures of different processes at W (111) are quite near. These experimental results were used to discuss the formation mechanism of single atom W tips.