This paper presents cost optimization analysis of reinforced concrete composite girders in RC pre-fabricated buildings designed according to the Egyptian Code of Practice (ECP 203-2017). The adopted objective function was to minimize the total cost of the RC pre-fabricated building including the cost of composite girders, columns and foundations considering the cost of the materials and labor for reinforcement, concrete and formwork. The structure is modeled and analyzed using a Microsoft Excel- 2013 spreadsheet. The optimization process was performed at three different levels. In the first level, optimum column arrangement was attained while the optimum dimensions for all RC elements of the selected structural plan of the pre-fabricated buildings were studied in the second level. In the third level, a comprehensive study was conducted in order to determine the optimum dimensions and reinforcement of RC composite girders by Generalized Reduced Gradient (GRG) method. Finally, the cost improvement of three buildings of composite concrete is demonstrated, and the results of the optimum and traditional design procedures are compared.