Crosslink density is one of the important parameters that govern the physical properties of fiber reinforced polymer (FRP) composites. Enhancement of crosslink density by effective matrix modification through nanoparticle incorporation is the most prominent way to improve mechanical and thermo-mechanical properties of FRP composites. In this study, at first, 0.1–0.4 wt.% amino-functionalized multi-walled carbon nanotubes (NH2-MWCNTs) were incorporated in SC-15 epoxy system and the variation in crosslink density was investigated using rubber elasticity theory. Subsequently, the effect of these MWCNTs on interlaminar shear strength (ILSS) of e-glass/epoxy composites was studied. Result obtained from the tests showed a linearly increasing trend in crosslink density and ILSS properties from 0 to 0.3 wt.% MWCNTs loading. Better dispersion of MWCNTs facilitated more crosslinking sites, whereas, the three-way reaction between amine functional groups of MWCNTs with epoxide groups of resin and epoxy silanes of fiber surfaces improved the crosslinking and thereby ILSS properties of e-glass/epoxy composites.