Mechanical properties of micro-diameter and medium-diameter steel fiber reinforced cement mortar were investigated.At a constant steel fiber volume fraction,the increase of mechanical properties of cement mortar reinforced by hybrid steel fiber can be larger than that by mono-diameter steel fiber.Different size hybrid steel fiber shows distinct hybrid effect on flexural strength of cement mortar.The m_S/m_B is an important factor for mechanical properties of hybrid steel fiber reinforced cement mortar.With increase of m_S/m_B,it needs more medium-diameter steel fiber to substitute micro-diameter steel fiber in hybrid steel fiber system.The hybrid systems of two steel fibers with different diameter have synergetic effect on fracture energy and toughness of cement mortar.