Nanostructured magnetic dots have been of great interest because of their technological importance in future high-density data storage devices. Deep understanding on the magnetization reversal process of nanoscale magnet is essentially important for such device operation, thus numerous studies have been reported so far for various materials with perpendicular magnetic anisotropy, such as Co/ (Pt and Pd) multilayers and CoPt alloys [1-3]. Through a large number of these efforts, it has been widely accepted that a nucleation of a reversed domain with a dimension of domain wall width initiates the magnetization reversal of the dot. However there are still many issues remained unclear. For example, where the nucleation site is, how the physical mechanism of nucleation is, and so on. Since the previously used polycrystalline dot array samples always contain structural and magnetic non-uniformity due to grain boundary and their distributions among the dots, a study using a single dot with epitaxial structure is essential to clarify these questions. In this study, we investigate the magnetization switching behavior by using single nanodots of epitaxially grown Co/Pt multilayer.