Atmospheric plasma treatment (APT) was used to surface-activate graphite nanoplatelets (GnP) as well as highly graphitic P100 fibers used to manufacture composites. X-ray photoelectron spectroscopy showed an increase in the O/C ratio of the treated surfaces when using either CO or O2 as the active gas, whereas CO exhibited less damage to the treated reinforcement carbon material. APT of P100 fibers resulted in a 75% increase in composite tensile strength when compared to composites using untreated fibers. Surface treatment of GnPs also resulted in GnP/epoxy composites with significantly higher glass transition temperatures (Tg's) and 50% higher flexural strengths than those with no surface treatment because of stronger particle-to-resin coupling, which was also evidenced by the fracture surfaces. The effect of GnP loading concentration and plasma treatment duration was also evaluated on the tensile strength of fiber-reinforced composites. The addition of untreated GnP filler resulted in a decrease in strength up to the 1% loading. However, higher loading conditions resulted in a 20% improvement because of GnP orientation effects. Fracture surfaces suggest that the fibers provided a mechanism for the GnPs to orient themselves parallel to the fiber axis, developing an oriented matrix microstructure that contributes to added crack deflection. Incorporating surface-treated GnPs in these composites resulted in tensile strengths that were as high as 50% stronger than the untreated systems for all loading conditions. Increased GnP-to-matrix bonding as well as enhanced orientation of the GnPs resulted in multifunctional composites with improved mechanical performance. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014, 131, 39994.