Aligned carbon nanotubes (CNTs) are being investigated for application to numerous material disciplines including structural composite materials due to their unique scaledependent physical properties. Prior work on nanoengineered composite architectures demonstrated improved interlaminar fracture toughness was dependent on epoxy type. In this work, we seek to understand the reinforcement mechanisms of aligned CNTs in a polymer matrix in the absence of fibers, of aligned polymer nanocomposites in different epoxy types. Preliminary investigations were performed using single edge notch beam specimens to isolate the fracture toughness of small CNT nanocomposites, and the mechanisms of reinforcement can be elucidated through tight control over CNT alignment and loading epoxy. While crack initiation off a sharpened precrack are unchanged with added aligned CNTs, an increase in fracture surface area and added crack arrest capability reveal mechanisms by which CNTs can add absorb energy and increase toughness post crack initiation. NomenclatureA-PNC = aligned CNT polymer nanocomposite CNT = carbon nanotube CVD = chemical vapor deposition DCB = double cantilever beam K Ic = Mode I critical stress intensity FFRP = fuzzy fiber reinforced plastic FRP = fiber reinforced plastic PNC = polymer nanocomposite RVE = representative volume element SEM = scanning electron microscope SENB = single edge notch beam V f = volume fraction I. Introduction arbon nanotubes (CNTs) are being investigated for application to numerous material disciplines including structural composite materials due to their unique scale-dependent and intrinsic physical properties. 1 With high specific strength and stiffness and nanoscale dimensions, 2 aligned CNTs are an attractive candidate for incorporation into laminated composites, including interlaminar reinforcement. Mechanical properties that are limited by the matrix-dominated interlaminar region in traditional composites are typically reinforced through changing the fiber architecture as in stitching/weaving or modifying the matrix properties through tougheners and additives. 3 Nanoengineered architectures integrate both approaches, reinforcing the interlaminar regions with nanoscale fibers that provide toughening through nanoscale fiber pull-out 4 and modifying matrix properties. As realized in Fuzzy Fiber Reinforced Plastics (FFRP) where aligned CNTs are grown radially on advanced fibers, the demonstrated improvements in Mode I toughness are shown to dependent on the type of epoxy and the CNT length through a combination of driving fracture towards or away from fiber surfaces and increasing fracture surface area. 5,6 The work here aims to decouple the multiscale effects in interlaminar toughening by determining the toughness of the