The flexible operations of gas turbine combined cycle power plants, especially with reduction of start-up time, are important. In this paper, the modeless start-up method without determination of the start-up curves corresponding to the start-up modes (e.g., hot start, warm start, and cold start) were developed. This method generates the start-up curves in which the start-up time is minimized against the initial steam turbine metal temperature by optimizing the start-up parameters. Based on the candidates of these start-up parameter values obtained during the optimization process, the look-up tables corresponding to the initial metal temperature for the parameters are estimated. The start-up time of the start-up curves generated by referring to the look-up tables continuously vary against the initial metal temperature without exceeding the thermal stress limit of the steam turbine rotor. The results obtained by the developed method show that modeless start-up curves for gas turbine combined cycle power plants can be generated. In addition, by visualizing the relationship of the start-up time and the initial metal temperature, the start-up curves can be interactively selected to facilitate planning of the plant start-up schedules.