In this work, the effect of acid treatments with H2SO4/HNO3 mixture of basalt fibers on mechanical interfacial properties of basalt fibers/epoxy composites was investigated. The surface properties of the basalt fibers were determined by their acid and base values, X-ray photoelectron spectroscope (XPS) analyses, and scanning electron microscope (SEM). The mechanical interfacial properties of the composites were assessed by interlaminar shear stress (ILSS) and critical stress intensity factor (KIC). The acid treatments led to a significant change of the surface characteristics of the basalt fibers. Keywords: basalt fibers, acid treatment, mechanical interfacial properties