The biaxial flexural response of strain-hardening UHPFRC circular slab element is investigated experimentally and analytically using the ring-on-ring test. The Digital Image Correlation (DIC) technology is applied to capture the 3D full-field of strain and deflection of the tensile surface through the whole loading process. Based on DIC analysis, a quasi-elastic limit is determined additionally to characterize the flexural response of UHPFRC under biaxial stress condition. The random and uniform distribution of matrix discontinuities in UHPFRC in the quasi-elastic domain, as well as multiple fictitious crack formation in hardening domain, is highlighted. In addition, ring-on-ring test and Four-Point Bending Test (4PBT) results are compared with special emphasis on flexural strength and characteristics of matrix discontinuity development. The results show higher ultimate flexural strength and energy absorption capacity at ultimate limit from the ring-on-ring test, and almost same flexural strength at the quasi-elastic limit for both tests. Finally, a simplified inverse analysis is proposed for the ring-on-ring test based on the elastic slab bending and yield line theories. The calculation results indicate a 19% lower tensile elastic limit stress of the UHPFRC, while an almost equivalent value of tensile strength is obtained from the ringon-ring test, compared with the 4PBT results. (C) 2020 Elsevier Ltd. All rights reserved.