This study fabricated nanocomposites with aligned graphene platelets and examined their mechanical properties. Both reduced graphene oxide (RGO) and functionalized graphene (FG) were used in this study. Graphene platelets were incorporated into epoxy matrix through mechanical mixing, followed by sonication for homogeneous dispersion. To align the graphene platelets, before curing, an electric field was applied to the epoxy–graphene compound. Real-time microscopic observation revealed that under an electric field, the graphene platelets rotated, translated, and eventually formed a chain-like microstructure along the electric field. Tensile tests were conducted on the nanocomposites with aligned graphene, and failure mechanisms were identified using scanning electrical microscopy of the fracture surfaces. For comparison, nanocomposites with randomly oriented graphene were prepared and then subjected to tensile loading. Moreover, the moduli of the graphene nanocomposites were characterized using the modified Mori–Tanaka micromechanical model. The experimental results indicated that the nanocomposites with aligned graphene exhibited higher tensile moduli and tensile strength than those with randomly oriented graphene. Moreover, the nanocomposites with FG had superior interfacial properties and thus superior mechanical properties compared with those with RGO.