In this study, hybrid utilization of high-strength steel in composite beams was proposed in order to maximize their flexural capacity and full-scale testing was conducted in two phases to investigate their flexural behavior. In fabricating specimens, high-strength steels were utilized for the bottom flange while normal-strength steels were used for the top flange and the web. In Phase I testing, however, all the high-strength steel bottom flange specimens were not able to reach their plastic moment by about 10–15% due to unexpected longitudinal shear failure along the beam axis, although sufficient shear studs were provided for full composite behavior and the plastic neutral axis location was limited within 15% of the total depth of the composite beam section. The specimens in Phase II testing designed with additional shear reinforcements showed no longitudinal shear cracking and developed their plastic capacity with reasonable deformability as intended in design. This implies that, different from the design of conventional composite beams, checking the longitudinal shear strength of composite concrete slab is crucial when designing hybrid composite beams utilizing high-strength steels. The nominal longitudinal shear strength was well predicted by the shear-friction based models in ACI 318-14 and AASHTO LRFD. The test results of this study show that when the limitation on the depth of the plastic neutral axis set forth by Eurocode 4 is satisfied along with sufficient longitudinal shear strength, the plastic stress design method can still be applied to the de-ign of hybrid composite beams utilizing high-strength steels whose nominal yield strength is as high as 6500MPa.