The growth of Co-silicide on two different kinds of Si(100) substrates covered with a chemical oxide was investigated. The off-axis Si(100), which was cut 4 degrees off to the (100) plane towards the [110] direction, and the on-axis Si(100), which was cut with less than 0.5 degrees, were prepared. The atomic scale microsteps of the two different Si(100) substrates were observed by high resolution transmission electron microscope. These Si substrates were chemically oxidized by submerging the Si wafers into a hydrogen peroxide (H2O2) solution for 10 min and loaded into an ultrahigh vacuum evaporation system. The 10-nm cobalt thin films were deposited on both kinds of substrates in this system and subsequently in-situ annealed for 10 min at temperatures between 400 and 700 degreesC with 100 degreesC increments. After this treatment, the Co-silicide is formed and analyzed by an X-ray diffractometer: scanning electron microscope, transmission electron microscope and Auger electron spectroscopy to verify the phase formation, surface and interface morphologies, and chemical composition. The Co2Si phase was found to be the first silicide formed during in-situ annealing. The formation temperature of CoSi2, on the 4 degrees off-axis Si(100) and on-axis Si(100) were 600 and 700 degreesC, respectively. This difference in formation temperature of the CoSi2 phase was considered to be related to the atomic scale microsteps of the Si surface.