In this study, the theoretical methodology is presented for estimating cost function based on operating modes for Combined Cycle Power Plant. The proposed method has estimated cost functions using the relationship between the gas turbine heat input and the output ratio of the steam turbine. In order to verify the proposed method, a regression analysis was performed using the single cost function estimated by the existing performance test method and the cost function for each operating mode estimated by the proposed method. The results of case studies using the 2016 generator input and output data are presented to show the effectiveness of the proposed method.