Three composite systems, having different interlaminar fracture toughnesses, were tested to characterize the matrix toughness effect on delamination growth behavior in composites under fatigue loading. With each composite system, three specimen types were tested: (1) double-cantilever-beam, (2) cracked-lap-shear, and (3) end-notched flexure specimens for mode I, mixed mode I-II, and mode II loadings, respectively. The measured delamination growth rate data were correlated with the corresponding strain energy release rates, GI, G(I−II), and GII. The cyclic delamination growth resistance of composites, expressed in terms of static interlaminar fracture toughness, that is, ΔG/GC, decreases with increasing matrix toughness. And this decrease depends on the loading mode. Further, this decrease in cyclic delamination growth resistance of composites, expressed in terms of normalized interlaminar fracture toughness (ΔG/GC), increases when fatigue loading mode is changed from mode I through mixed mode I-II to pure mode II.