The oxidation of Co film on Si and SiO2 substrates during heat treatment was investigated by TEM, HREM and XRD. The results show that the cobalt oxide on Si and SiO2 substrates, annealed below 550 degrees C, is found to be CoO instead of Co3O4. Analysis showed that the annealing ambient affected the results. Oxidation of Co films also has an effect on the silicide formation at the Co/Si interface, and it leads to CoSi2 formation at a lower temperature than without oxidation. We also confirmed that CoO formed before annealing at 550 degrees C can completely change into silicide after further heating under vacuum at 900 degrees C for 5 min.