Abstract: To promote the application of ultra-high toughness cementitious composites (UHTCC) in composite structures, research on mechanical performance of steel reinforced UHTCC (SRU) beams was carried out. The load-deflection relationship, the flexural capacity, the crack development, the failure mode and the deformation coordination of the steel and two kinds of cement-based materials were compared and analyzed with steel reinforced concrete (SRC) beams. The results indicated that at approximately same uniaxial compressive strength of UHTCC and concrete, the deformation capacity difference between UHTCC and concrete resulted in obvious mechanical performance differences between the two kinds of composite beams. The maximum deflection in midspan of the SRU beam with better integrity was 2.26 times of the SRC beam. The SRU beam crack width at the same load was only about 50% of the SRC beam. The SRU beam flexural capacity increased by 6.4% and the midspan cross-section strain was more in line with cross-section assumption. UHTCC can coordinate with the steel deformation and work together with steel.