The real-time monitoring method of the Cu/In ratio in the bilayer deposition process of CuInSe2 thin film was originally established and demonstrated by applying the bilayer process. The principles of this method were based on the idea that the hole concentration of the film was abruptly changed at the stoichiometric composition while the film composition gradually changed from Cu rich to In rich through the stoichiometric composition during the deposition process. This method was very useful for the composition control of the CuInSe2 film.