Abstract: On the basis of the four point bending experiments of beams and slabs of hybrid fiber reinforced cementitious composites with CaCO₃ whisker, a mathematical formula was proposed to determine the relationship between flexural toughness index and fiber reinforcing index (S). The S takes into account of the effect of the fiber tensile strength and mechanical anchorage on the flexural tensile properties of hybrid fiber/concrete, which has clear physical meaning. The quadratic function can reflect the toughness reinforcing effect of hybrid fiber system on the hybrid fiber/concrete, and it can be used to optimize fiber composition by determining the extreme value of the quadratic function. The mathematical model has good applicability on steel-synthetic and steel-plant fibers reinforced concrete, without considering the matrix (mortar or concrete) and specimen shape (beam or slab). In addition, the formula can be applied not only to the flexural toughness and equivalent flexural strength, but also to the flexural toughness index based on ASTM C1018, e.g. I₅, I₁₀, I₃₀ and I₅₀.