In this work, red mud (RM) was chemically modified by 0.1, 1, and 5 M H3PO4 solution to prepare epoxy/RM nanocomposites. The effect of chemical treatment on a RM surface was studied in terms of pH, acid-base values, N-2/77 K gas adsorption, equilibrium spreading pressure (ne), and surface free energy. The mechanical interfacial properties of epoxy/RM nanocomposites was measured in the context of critical stress intensity factor (K-IC) and critical strain energy release rate (G(IC)) measurements. From the experimental results, the acidic surface treatment led to a modification of the RM surface properties, such as the surface acid values, porosity, specific surface area, ire, and London dispersive component (gamma(S)(L)) of surface free energy of the treated RM as an increase of the treatment concentration. In the fracture toughness (KIC and GIC) measurements, the mechanical interfacial properties of epoxy/treated RM nanocomposites were intimately correlated with the improvement of interfacial adhesion between the RM surface and a matrix and the increase of gamma(S)(L) of surface free energy of the RM due to the development of S-BET or porosity of RM surfaces.