Laminated composites mostly suffer from layer separation and/or delamination, which may affect the stiffness, strength and lifetime of structures. In this study, we aim to produce micron-scale thin carbon nanotubes (CNTs) reinforced adhesive nanofibrous interleaves and to explore their effectiveness when incorporated into structural composites. Neat polyvinyl butyral (PVB) and solutions containing low fractions of CNTs from 0.5 to 2 wt.% were electrospun directly onto carbon fiber prepregs. These interlayered laminates were cured above the glass transition temperature (Tg) of PVB to achieve strong interlaminar binding and also to resist crack re-initiation. The effect of CNTs presence and their mass fractions both on total Mixed-Mode I + II fracture toughness (GC) and crack length was investigated under Mixed-Mode I + II loading. Almost 2-fold increase in GC was reported in interlayered composites compared to non-interlayered laminates, associated to toughening effect of adhesive PVB/CNTs nanofibrous interlayers. Furthermore, the post-fracture analysis revealed the aid of CNTs interleaves in retarding delamination and afterward stabilization of crack propagation.