The behavior of single Ir adatoms and small Ir clusters on the Ir (111) and (001) surfaces such as the surface diffusion, the structure transformation of clusters, the dissolution of the top surface layers etc. as functions of the temperature has been studied with the field ion microscope. From these experiments, diffusion parameters and the dissociation energy of plane edge atoms etc. have been derived. We have also observed a 'strange behavior' in the temperature dependence of cluster structure transformation which disagrees with a simple consideration of the binding energy difference of the two structures.