Fibers are incorporated into cementitious matrices to help control cracking by bridging and providing load transfer across cracks and pores. Typical reinforcement of concrete is usually provided at the macro- and microscales by using macrofibers and microfibers, respectively. Although microfibers delay the propagation of microcracks, they do not stop their initiation. Theory suggests that nanofibers delay the formation of nanocracks, thus requiring higher loads to initiate cracking, which improves the weak tensile strength of the cementitious matrix. In this paper, a detailed investigation of the effects of carbon nanofibers (CNFs) on the flexural strength and nanostructure of the cement matrix was conducted. An ultra-high-resolution field emission scanning electron microscope was used to investigate the morphology of the nanocomposites. Nanoimaging of the fracture surfaces of cementitious nanocomposites reinforced with CNFs at different concentrations has shown that CNFs are able to bridge nanocracks and pores and achieve good bonding with the cement hydration products. As a result, the incorporation of CNFs was shown to improve significantly the flexural strength of the cementitious matrix.