Abstract: To investigate the size effect on the flexural fatigue performance of Ultra-High Performance Concrete (UHPC), UHPC reinforced with 2% volume fraction of straight steel fibers was used to prepare two sizes of specimens with dimensions of 40 mm×40 mm×160 mm and 100 mm×100 mm×400 mm. Both types of specimens were subjected to flexural fatigue loading tests at different stress levels. The results show that the static flexural strengths of the 40 mm specimens and the 100 mm specimens are 31.64 MPa and 28.53 MPa, and their flexural fatigue strength ratios range from 0.7-0.75 and 0.63-0.68, respectively. The deformation development of large-size specimens with fatigue failure exhibits a typical three-stage evolution, while no obvious third stage appears in the deformation curves of small-size specimens. With the increase of UHPC specimen size, the fatigue strength decreases accordingly. Moreover, the size effect of flexural fatigue strength is more significant than that of the static flexural strength. A database of UHPC flexural fatigue tests was established. Based on this database, a UHPC flexural fatigue equation considering the size effect was proposed, which can be used to predict the fatigue life of UHPC with 2% volume fraction of straight steel fibers for different specimen sizes. The critical size for the size effect of UHPC flexural fatigue strength was discussed.