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Repowering as an experimented & generable way can be an effective method for reconstructing steam power plants. This method up to the steam power plants properties can be done in different methods. In this paper overall conditions have been considered for doing a suitable method between different methods. According to selected method, techno-economical parameters of obtained cycle are given. In other hands, according to the considerable value of future investment for constructing gas turbine plants, we compare the generated electricitys cost of this type of power plants with repowering methods cost. The base of doing this calculation is the similar costs in combined cycle power plants. Therefore we obtain a scale in order to compare generated electricity cost in three scenarios of steam power plant repowering, building gas turbine plant and combined cycle power plant.