There are many civil engineering infrastructures that cannot fulfill their intended purpose. Demand for higher load carrying capacity, material degradation, or change in use are the main reasons for such problems. In these conditions there are some definite structural strengthening techniques with new composite materials. Rehabilitation is a very important strategy to extend the service life of existing Reinforced Concrete (RC) members, otherwise the structure has to be demolished and reconstructed. It is more economical to strengthen or repair structures compared to demolition and reconstruction with additional advantages of better energy saving as well as sustainability of resources. The aim of this study was to develop an effective, economical, and practically applicable Engineered Cementitious Composites (ECC) which can be used to strengthen RC beams in flexure. In this study experimental investigation was carried out on the flexural performance of RC beams strengthened by ECC. Under-reinforced beams were strengthened in flexure by providing ECC layer on the tensile side of RC beams. Specimens of RC beams strengthened by ECC with steel fiber of aspect ratio 31.64 but different fiber volume (1%, 1.5%, 2%, and 2.5%) and the control under-reinforced beam were prepared. Comparison on the performance of those RC beams strengthened by ECC and the control sample has done. ECC with steel fiber of 1.5% by volume came out to be the optimum mortar mixture as flexural strengthening material and the beam strengthened by it has highest load carrying capacity compared to the remaining beams. It was found that the specimen strengthened with the optimum ECC satisfied the objectives of this study.