<p>Hydroxyl-functionalized graphene nanoplatelets (GOH) were incorporated into an epoxy resin at concentrations of 0.1, 0.5, 1 and 3 parts per hundred resin (phr). The curing system, based on dicyandiamide as a solid curing agent and 1-methylimidazole as an accelerator, was homogeneously mixed. The curing behavior of the resulting nanocomposites was evaluated using differential scanning calorimetry (DSC). Unidirectional carbon fiber/epoxy nanocomposites were fabricated through hot pressing after fiber impregnation with the modified resin. Mechanical testing was conducted to assess the transverse tensile strength of the unidirectional carbon fiber-reinforced epoxy (UCFRE) composites and to compare the experimental results with theoretical predictions. Further evaluations included flexural strength, fracture toughness, interlaminar shear strength (ILSS), and lap shear strength (LSS). Additionally, the synergistic effect of a hybrid nanoparticle system consisting of 0.4 phr GOH and 0.1 phr carboxyl-functionalized multi-walled carbon nanotubes (MWCNTs) on composite performance was investigated. The results showed that adding 1 phr GOH improved the transverse tensile strength and strain-at-break by 17.6% and 20%, respectively. A more pronounced enhancement (60%) in transverse strain-at-break was observed at 0.5 phr GOH. Furthermore, at 1 phr GOH, longitudinal flexural strength, deflection-at-break, and flexural modulus increased by 6.8%, 6.9% and 6.9%, respectively. The incorporation of 3 phr GOH significantly increased fracture toughness by 71% in the transverse direction and 42% in the longitudinal direction.</p> Graphical abstract <p></p>