This paper investigates experimentally the biaxial flexural fatigue behavior of thin circular slab specimens made of Ultra High Performance Fibre Reinforced Cementitious Composite (UHPFRC), using ring-on-ring test method. The circular slab specimens represent an external tensile reinforcement layer for reinforced concrete (RC) slabs or a new thin slab element for the application, e.g. in bridge construction. Four series of flexural fatigue tests under constant amplitude fatigue cycles up to 20 million cycles are conducted with varying S (stress level, the ratio of the maximum fatigue force to the load-carrying capacity of the slab specimen) ranging from 0.45 to 0.80, targeting at the endurance limit of UHPFRC material fatigue damage propagation under equi-biaxial stress condition. By means of digital image correlation (DIC) system, the tensile surface of each specimen is monitored continually, and several fatigue damage parameters are studied for the whole testing, including deflection, multiple micro-cracking, fictitious cracks (width and length), stiffness and dissipated energy.