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This study experimentally investigates the aged tensile strength behavior and microstructure of the ultra-high-performance fiber-reinforced concrete (UHPFRC) fabricated with an optimal ratio between the concrete and steel fibers. In the mixing process, a superplasticizer is added to enhance the tensile strength with the increasing age of the material. The appropriate addition of the superplasticizer is extremely crucial because it significantly affects the workability, durability, and tensile strength of the UHPFRC. In this work, the volumetric fraction of the steel fibers is determined to achieve the target splitting tensile strength of 25 N/mm 2 at elevated temperatures in which several properties of the concrete are degraded. It is shown that the proposed UHPFRC can achieve the target tensile strength in 28 days. To better understand this tensile strength behavior, a microstructure analysis is also conducted using scanning electron microscope images. From this analysis, it is determined that micro-silica is involved in imparting high density to the microstructure and increases the packing density of the compounds. This phenomenon directly influences the increment in the tensile strength of the proposed UHPFRC.