The objective of this investigation is to evaluate the effect of functionalized carbon nanotubes on the delamination behavior of S2-glass fiber/ multi-walled carbon nanotubes (MWCNTs)/epoxy hybrid composites. Interlaminar fracture toughness (IFT) specimens were made by wet filament winding using S-glass fiber and epoxy matrix. Different types of functionalized MWCNTs, a carboxyl functionalized carbon nanotube (COOH-CNT) with two different initial length distributions and a silane coupling agent functionalized carbon nanotube, were added into the epoxy material. The MWCNTs modified epoxy were used either as the matrix material for filament winding composite plies or used as resin interlayer material. The mode I and mode II quasi-static interlaminar fracture behaviors were characterized using the double cantilever beam and end-notched flexure tests, respectively. The interlaminar fracture test results were used to evaluate: 1) the effect of adding functionalized MWCNTs; 2) the effect of changing MWCNT types, aspect ratio, and concentration; and 3) the effect of adding a resin interlayer at the fracture surface on mode I and mode II fracture toughness. Compared to the baseline material, the mode I onset and propagation fracture toughness was increased by 109-139% by adding 1 weight percent (wt%) short COOH-CNTs into the matrix in the fiber reinforced plies and the resin interlayer at fracture interface. The highest mode II fracture toughness among laminates investigated (49% improvement compared to the baseline material) was achieved by adding 0.5 wt% COOH-CNTs into the fiber reinforced plies and 0.5 wt% long COOH-CNTs into the resin interlayer at fracture interface. Scanning electron microscope images of CNT-filled epoxy showed possible toughening mechanisms achieved by crack pinning and crack deflection by functionalized CNTs.