The objective of this study is to investigate the flexural behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) beams reinforced with internal glass fiber-reinforced polymer (GFRP) bars. Four large UHPFRC beams with different reinforcement ratios (ρ = 0.53%, 0.79%, 1.41%, and 1.71%) were fabricated and tested. This study investigated the effects of the GFRP reinforcement ratio on the load carrying capacity, deflection at service loads, cracking behavior, and deformability. Test results showed that higher load carrying capacity, higher deformability, and lower crack width were obtained as greater GFRP reinforcement ratios were used. The post-cracking stiffness before crack localization was substantially improved because of the strain-hardening response of UHPFRC. Finally, the service deflections were compared with the predictions of the ACI 440.1R and Bischoff's model.