Amino functionalized multiwalled carbon nanotubes (A-MWCNTs) reinforced two phase (A-MWNT-epoxy) and three phase (A-MWCNTs-carbon fiber-epoxy) nanocomposites were fabricated with 0.25 wt%, 0.5 wt% and 1.0 wt% loadings of A-MWCNTs. It is observed that, A-MWCNTs can improve the crosslink density of epoxy significantly. Fracture toughness of epoxy matrix is found to increase up to an optimum crosslink density improvement, indicating the role of crosslink density in imparting toughness to epoxy apart from the crack deflection contributions of A-MWCNTs. In addition to that, this study infers that, tensile, flexural properties of the three phase composites are strongly influenced by the fracture toughness changes of the matrices. This study, thus proposes additional mechanisms of toughness enhancements for two phase and mechanical properties enhancements for three phase composites imparted by A-MWCNTs. © 2012 Elsevier Ltd. All rights reserved.