The biaxial flexural fatigue behavior of thin slab elements made of strain-hardening Ultra High Performance Fiber Reinforced Cementitious Composite (UHPFRC) is investigated experimentally by means of the ring-on-ring test method. Fourteen flexural fatigue tests under constant amplitude fatigue cycles up to the Very High Cycle Fatigue domain (20 million cycles) are conducted with varying maximum fatigue stress level S ranging from 0.50 to 0.68. Digital Image Correlation (DIC) technology is applied to capture the 3D full-field strain contours on the tensile surface through the entire fatigue test. Test results presented in the S-N diagram reveal a fatigue endurance limit under biaxial flexural fatigue at S = 0.54. Fatigue tests exhibiting failure show four distinct phases of damage evolution, while only the first two phases are observed in the case of run-out tests. DIC analysis reveal formation and propagation of multiple fine fictitious cracks that dominate the stable fatigue propagation phase with slow rate, representing the longest part of fatigue life of the UHPFRC specimen. Finally, the secant modulus of deflection and fictitious crack opening with respect to fatigue cycles is found to characterize quantitatively fatigue damage evolution.