The large specific surface area, relatively active surface chemistry and close spacing of carbon nanofibers (CNFs) make them particularly effective in controlling the inception and growth of microcracks in cementitious materials. Given the fine geometry of nanofibers, their interactions with cementitious matrices would benefit from increased packing density and reduced size of pore system in matrix. These favorable features for effective interactions with CNFs are provided by high-strength (HSC) and especially ultra-high-performance concrete (UHPC). An optimization experimental program was conducted in order to identify the desired combination of nano-/micro-scale reinforcement. The performance efficiencies of the desired nano-/micro-scale (hybrid) reinforcement in HSC and UHPC were evaluated and compared. The desired nano-/micro-scale reinforcement system comprising CNF and PVA fiber generally produced higher relative gains in the engineering properties of UHPC than HSC. Two different (PVA and steel) fibers were evaluated in conjunction with CNF in UHPC. The desired combination of steel fiber with CNF had higher fiber volume fraction than the desired combination of steel fiber with CNF. The desired nano-/micro-scale reinforcement with steel fiber performed better than that with PVA fiber in HSC.