Abstract: In steel reinforced ultra high toughness cementitious composites (RUHTCC) flexural beam, the random-oriented PVA short fiber reinforced cementitious composite with ultra high toughness (UHTCC) is used to replace conventional plain concrete or fiber-reinforced concrete materials which behave in strain-softening quasi-brittle manner, thus the ductility and damage tolerance of RUHTCC structures are enhanced and thereby they are regarded as a new form of structural member with excellent seismic resistance behaviors. Apart from the reinforcement ratio and the tension and compression mechanical properties of UHTCC, cross-sectional size of the beam also is an important parameter influencing the flexural behavior of RUHTCC beam. Based on previous research achievements, beams with various cross-sectional sizes with varying beam depth, width and cross-sectional area were investigated to study their flexural responses. The results revealed that the depth of beam has more significant influence on the flexural capacity than the beam width, and that the degree of improvement on flexural capacity and deformation decreases with the increase in the depth of beam but unchangeable with the increase in the beam width. It was also discovered that cross sectional size of beam has almost no impact on the crack width as long as the strain in the bottom surface of beam is below the ultimate tensile strain of UHTCC materials used. Subsequently, some suggestions for the flexural design of RUHTCC beam were recommended.