In recent decades, scientists and engineers have developed Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) as a repair technology to increase the punching shear and fatigue performance of damaged RC slabs. To date, the experimental and numerical studies of damaged RC slabs repaired with UHPFRC have been conducted, and they showed significant improvements in the fatigue behavior of the repaired RC slabs. However, there are some cases where the traffic has to be reopened shortly after the casting of UHPFRC. There is a possibility that the developed stiffness of UHPFRC during its early age is insufficient to resist the traffic load. Therefore, this study investigates the behavior of RC slabs repaired with early-age UHPFRC under fatigue loading. The 3-D finite element analysis software is used to conduct the fatigue analysis of the repaired RC slabs with the 1-day, 7-day, and 28-day-old UHPFRC. The Young’s modulus is reduced to represent the insufficient hydration development in the corresponding ages of UHPFRC. The fatigue behavior of the RC slabs under the moving wheel load is investigated by employing bridging stress degradation concept. The results of this study show that in the 1-day and 7-day-old early-age UHPFRC repair, center displacement and crack propagation increase higher than in the 28-day-old UHPFRC repair. However, they are still significantly lower compared to those of the unrepaired RC slab, hence verifying the improvement of fatigue behavior of the repaired RC slabs with the early-age UHPFRC. This conclusion provides a quantitative evaluation of fatigue behavior of the repaired RC slabs with UHPFRC by considering the early-age stiffness.