In this study, the effects of calcined diatomaceous earth (CDE), polypropylene fiber (PF), and glass fiber (GF) on the mechanical properties of ultra-high-performance fiber-reinforced concrete (UHPFRC) were observed, and a total of 33 UHPFRC mixtures, consisting of 3 mixtures without fiber, 15 mixtures with PF, and 15 mixtures with GF were prepared. Subsequently, the fresh concrete mixtures were tested for flow, while the hardened concrete specimen’s mechanical properties were analyzed. These tests include compression, splitting tensile, and flexural tests. The test results showed that the use of 5 and 10% CDE as a binder for cement replacement improved the compressive strength, splitting tensile strength, and flexural strength of the UHPFRC. Furthermore, the addition of PF and GF contents of up to 1% of the concrete volume increased the compressive strength of the UHPFRC, while their contents of up to 1.5% improved their splitting tensile strength and flexural strength. It is also important to note that the workability of the UHPFRC reduced as the fiber and CDE contents increased. Finally, based on the experimental data tested in this study, the relationship between splitting tensile strength, flexural strength, and compressive strength of the UHPFRC containing PF and GF were proposed. Moreover, the reduction in flow value, which is a function of the volumetric content of both PF and GF, with the CDE contents was also proposed.