The objective of this investigation is to evaluate the fatigue delamination behavior of S-glass fiber composites toughened by multi-walled carbon nanotubes (MWCNTs). Unidirectionally reinforced interlaminar fracture toughness (IFT) specimens were made by wet filament winding using S-glass fiber and epoxy matrix. Two types of epoxy matrix materials are evaluated - rigid and flexible. Each of the composites is tested with and without MWCNTs added to the matrix of selected layers of at a concentration of 0.5 weight percent. The MWCNTs were previously observed to increase both mode I and mode II delamination resistance of the rigid matrix system composite but reduce delamination resistance of the flexible matrix system. In this work, mode I and mode II interlaminar fracture behaviors in cyclic loading were characterized using double cantilever beam and end-notched flexure tests, respectively. For each type of test, a power law relation was used to fit the crack growth rate vs. the maximum strain energy release rate (SERR) data. From mode I testing, reduction in the overall crack growth rate was observed comparing the MWCNTs modified rigid to the regular rigid composite, however no such effect was observed for the flexible resin system. From mode II testing, the exponent of the power law relation was found to be decreased by adding MWCNTs into the rigid resin composite, however no significant change of the exponent was observed by adding MWCNTs into the flexible resin system.