There is an increasing demand for high performance composites with enhanced mechanical and electrical properties. Carbon nanofibres offer a promising solution but their effectiveness has been limited by difficulty in achieving directional alignment. Here we report the use of an alternating current (AC) electric field to align carbon nanofibres in an epoxy. During the cure process of an epoxy resin, carbon nanofibres (CNFs) are observed to rotate and align with the applied electric field, forming a chain-like structure. The fracture energies of the resultant epoxy nanocomposites containing different concentrations of CNFs (up to 1.6 wt%) are measured using double cantilever beam specimens. The results show that the addition of 1.6 wt% of aligned CNFs increases the electrical conductivity of such nanocomposites by about seven orders of magnitudes to 10−2 S/m and increases the fracture energy, GIc, by about 1600% from 134 to 2345 J/m2. A modelling technique is presented to quantify this major increase in the fracture energy with aligned CNFs. The results of this research open up new opportunities to create multi-scale composites with greatly enhanced multifunctional properties.