The use of carbon nanotubes (CNTs) as a reinforcing material in a polymer matrix has in-creased in various industries. In this study, the flexuralbehavior of CNT-modifiedepoxy/basalt (CNT/epoxy/basalt) composites is investigated. The effects of CNT modificationwith silane on the flexuralproperties of CNT/epoxy/basalt composites were also examined. Flexural tests were performed using epoxy/basalt, oxidized CNT/epoxy/basalt, and silanized CNT/epoxy/basalt multi-scale composites. After the flexuraltests, the fracture surfaces of the specimens were examined via scanning electron microscopy (SEM) to investigate the fracture mechanisms of the CNT/epoxy/basalt multi-scale composites with respect to the CNT modificatio process. The flexuralproperties of the epoxy/basalt composites were im-proved by the addition of CNTs. The flexuralmodulus and strength of the silane-treated CNT/epoxy/basalt multi-scale composites increased by approximately 54% and 34%, re-spectively, compared to those of epoxy/basalt composites. A SEM examination of the frac-ture surfaces revealed that the improvement in the flexuralproperties of the silane-treated CNT/epoxy/basalt multi-scale composites could be attributed to the improved dispersion of the CNTs in the epoxy.