Abstract: Four-point bending tests were performed on ultrahigh toughness cementitious composite(UHTCC)specimens for the purpose of understanding the influence of specimens’ thickness on its flexural behavior. The further explanation on this influence was made through theoretical dimension analysis. In the evaluation of toughness property, a new parameter, defined as energy dissipation per unit volume of plastic hinge region was proposed. The experimental results show that, the nominal flexural strength-flexural strain curves and crack width evolution curves of different thickness specimens are almost identical. Mid-span deflection decreases in an inversely proportional manner with the increase in specimen’s thickness. The ratio of neutral axis depth to specimens’ thickness at ultimate failure nearly keeps a constant value(about 0.88). Toughness index calculated based on ASTM C 1609 standard rises as the specimens’ thickness increases, whereas new proposed toughness index T_v is independent of specimens’ thickness. Therefore, for ultrahigh toughness cementitious composite exhibiting tensile strain-hardening characteristic, flexural strength, toughness capacity and crack width have no size effect. With these findings, standard specimen dimension, i.e., 100 mm×100 mm×400 mm specified for traditional FRC, is suggested for the flexural test of ultrahigh toughness cementitious composite.