Influences of initial surface states on formation processes of epitaxial CoSi2(100) films on Si(100) surfaces have been investigated by scanning tunneling microscopy. Epitaxial CoSi2(100) films were formed on various initial Si surfaces by solid-phase epitaxy of 3-ML-thick Co films. The CoSi2(100) films formed on amorphous, defect-induced and Si-island-formed Si surfaces show the increase in the areal ratio and the reduction of the height of the film. These facts suggest that surface defects and steps act as nucleation sites of CoSi2(100) and enhance the multiple nucleation and layer growth. As a result, the agglomeration of CoSi2 films would be suppressed and the quality of CoSi2(100) films is improved. In addition, it is clearly found that an almost pinhole-free and atomically-flat silicide film can be realized on the oxygen-adsorbed surface.