A waffle-shaped UHPFRC deck slab has been designed and tested by running wheel fatigue tests to assess its fatigue resistance under traffic loading within the intended operational period. However, these tests did not result into destructive outcomes, and the specific failure mode remains undisclosed. Consequently, constitutive laws of the UHPFRC have been experimentally identified to elucidate the failure mechanism and the progression of damage in the UHPFRC deck slab by Non-Liner Finite Element Method (NLFEM) analysis. In this paper, various fatigue loading tests employing UHPFRC specimens were conducted to comprehend the evolution of the mechanical characteristics of the material under fatigue. The results indicated a reduction in the elastic modulus of the UHPFRC and an increase in displacement corresponding to an increase in the number of fatigue loading, thus signifying the potential for establishing these shifts of mechanical properties as governing parameters for fatigue dependent constitutive laws.