Abstract Today’s demand for environmentally friendly and energy-efficient solutions to the construction industry has driven researchers to match natural resources with traditional technics to develop new building technologies. However, there are literature limitations about the correlation of fiber-matrix interface with the failure of natural fibers in mortar plates, which hinders the advances in understanding the mechanical properties of these composites on a structural scale. The present work investigated the mechanical behavior and fractography of cement-based composites reinforced by natural piassava and jute fibers. The experimental program included flexural tests and scanning electron microscopy analyzes. The developed composite material under flexural tests demonstrated a flexural-softening behavior, reaching up to 5.7 MPa, with a considerable residual strength ruled by toughness. The fractography analyses presented the fibers’ structure after mechanical tests and how effective its interaction with the matrix was. The piassava fibers demonstrated significant adherence when favorably oriented, while jute fibers (used as twisted yarn) provided voids in the composite by its partial matrix-covered filaments.