The solid-phase reactive epitaxial growth processes and structures of Er/Si(100) thin films were investigated by coaxial impact-collision ion scattering spectroscopy (CAICISS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The as-deposited Er film grown at room temperature was transformed into crystalline rectangular-shaped islands after annealing at 900 °C. These islands have a hexagonal AlB 2 -type structure and the epitaxial relationship is determined to be ErSi 2 (011¯0)[0001]//Si(100)[011¯]. It has been revealed that the surface of the Er silicide island is terminated with an Er plane.