The effectiveness of electric field alignment of graphene nanoplatelets (GNPs) in increasing the fatigue resistance of epoxy nanocomposites is investigated. Aligning the GNPs using an electric field yields a greater improvement in the fatigue crack growth resistance of the epoxy than obtained using randomly-orientated GNPs, particularly in the near threshold region. The improvement was due to several toughening mechanisms which retard the fatigue crack growth in the epoxy nanocomposites. These toughening mechanisms become more active when the GNPs are aligned normal to the direction of fatigue crack growth, which results in a higher fatigue resistance for the epoxy nanocomposites containing aligned GNPs than for those containing randomly-orientated GNPs.