The objective of this investigation is to compare two means of increasing the interlaminar fracture toughness (IFT) of fiber composites made by filament winding. Unidirectionally reinforced IFT specimens were made by wet winding using a baseline rigid epoxy resin modified either with flexiblizer or multi-walled carbon nanotubes (MWCNTs). The Mode I and Mode II IFTs of laminates with various combinations of MWCNTs and flexiblized epoxy on either side of the fracture plane increased substantially in comparison to the control laminate having allrigid epoxy. Placing a flexible epoxy layer on just one side of the fracture plane eliminated at least half the potential gain of using flexible epoxy on both sides of the fracture plane. Using flexible epoxy had a stronger influence on Mode I behavior than Mode II behavior. Introducing a small amount of MWCNTs to the rigid epoxy on both sides of the fracture plane has less of a benefit than using flexible resin in the Mode I case. However, in Mode II, using MWCNTs in the rigid resin was nearly as effective in toughening the material as using flexiblizer on both sides of the fracture plane.