The formation mechanism of epitaxial $CoSi_2$film using Co/Ti/Si system have been investigated in terms of the thickness of an intermediate Ti layer and the annealing ambient, which vary the reaction barrier that controls the Co flux. In the case of thin Ti interlayer and annealing in nonreactive ambient(vacuum), the reaction barrier formed Ti interlayer is Ti(O, C) layer. Howeverm, in the presence of driving force of Ti outdiffusion(annealing in nitrogen ambient), the Ti interlayer transforms to the ternary silicide as reaction barrier due to the advanced Ti outdiffusion. These reaction barriers limit the Co flux and lead to the formation of $CoSi_2$ without the formation of an intermediate $Co-silicide(Co_2Si, ;CoSi)$.