The evolution of the ultra-thin Co films on Si(lll) substrates upon annealing temperature in the range from room temperature to 800 degrees C was studied by means of photoemission spectroscopy (PES) with synchrotron radiation and low energy electron diffraction (LEED) techniques. 0.2-1.2 Ra. Co was evaporated onto Si(lll)-7 X 7 at room temperature. The behavior of the Go-silicon interfaces upon the annealing temperature was examined using Si 2p core level shifts and valence band measurements. The Si 2p core level spectra were taken with the photon energy of 130 eV and we observed the core level shift upon the formations of CsCl-type CoSi and the CaF2 type CoSi2 as a function of annealing temperature and of the thickness of Co. We also estimated the diffuse depth of Co depending on the annealing temperature.