By introducing a concept of condensing potential, a theoretical method is established to predict the geometric structure of two-dimensional （2D） adatom islands on crystal surface. Calculations based on semi-empirical potentials （surface-embedded-atom method potential and O. Johnson potential） show that on Cu（111） and Ag（111） surfaces the homoepitaxial 2D islands take on hexagons, while on Pt（111） surface the islands are truncated triangles, and on Cu（100） and Ag（100） surfaces the islands have square shape. These theoretical predictions are in good accordance with the experimental observations . Since calculation of the condensing potentials is independent of the size of islands, this theoretical model could be widely used to predict the shapes of 2D islands on various surfaces.