FRP bars are considered alternatives to steel bars for reinforcing concrete structures in harsh environments. FRP bars are non-corrosive, light weight, non-magnetic and have high longitudinal strength and low thermal and electric conductivity. This paper experimentally investigated the flexural behaviour of high strength concrete (HSC) and ultra-high strength concrete (UHSC) beams reinforced with glass fiber reinforced polymer (GFRP) bars that has not been addressed in the literature before. Beams of 2400 mm long, 100 mm wide and 150 mm high were tested under quasi-static loading (three point loading). Influence of reinforcement ratio and compressive strength of concrete (HSC and UHSC) on the load carrying capacity, deflection, energy absorption, strains in the concrete and reinforcement, and failure modes were investigated. Test results found that over-reinforced HSC and UHSC GFRP bar reinforced concrete (GFRP-RC) beams showed an amount of pseudo ¿ductility¿ compared to under-reinforced HSC and UHSC GFRP-RC beams, where failure was brittle, without any prior warning. Energy absorption capacities were found to be higher for UHSC GFRP-RC beams for the same amount of reinforcement compared to HSC GFRP-RC beams. FRP design recommendations in ACI (2015) and CSA (2012) were compared with experimental data. FRP design recommendations for the calculation of flexural strength were found to be conservative (load-carrying capacity was under-predicted by 36% for both HSC GFRP-RC beams and UHSC GFRP-RC beams). However, FRP design recommendations for the calculation of deflection at the load carrying capacity were found to be un-conservative (deflections were under-predicted by an average of 10-22% for the HSC GFRP-RC beams and UHSC GFRP-RC beams).