Structural evolution of Co/Ag/Ge(111) at high temperatures was studied by using scanning tunneling microscopy and low energy electron diffraction. The √3 × √3-Ag layer between the substrate Ge(111) and Co adatoms can avoid the formation of Co-Ge compounds below 800 K. The Co atoms nucleate to form islands where √13 × √13 or 2 × 2 reconstructions were observed after annealing between 373 K and 737 K. The √13 × √13 structure with mirror symmetry relative to [-211], [11-2], and [1-21] axes was observed for 1-2 layer Co islands. Co islands with over 2 layers appear 2×2 structure. All reconstruction structures of the nano-sized Co islands and substrate Ag/Ge(111) √3 × √3 surface were analyzed using the atomic hard sphere model. The bright protrusions of these reconstructions all sit in the centers of Ag or Ge trimers, which were predicted to have maximum binding energy.