The effect of interleaf layers incorporated between laminae on the mechanical properties of quasi-isotropic carbon/epoxy laminates was investigated. Two kinds of laminates, with and without interlayers, were compared in this study. Static and fatigue tests in tension, and vibration damping tests were performed on both laminates. It is shown that introducing the interleaf layers remarkably improved the ultimate tensile load and tension fatigue resistance of the laminates. The interlayers delayed the onset of interlaminar delamination from the free edge and impeded the progression of interlaminar delamination until final failure. It is also shown that the vibration damping of CFRP laminates can be remarkably improved by introducing compliant interlayers between composite plies. The loss factors of interleaved CFRP were 5 through 50 times as large as those for conventional CFRP in the case of quasi-isotropic laminate configuration, which depend upon the location of interlayers between composite plies. Discussion is also made on the optimum interleaf width in the case when the interleaf films were placed only in a limited local area near free edges, in order to tailor the interfacial properties and thus improve the fatigue resistance of quasi-isotropic carbon/epoxy laminatesinterlaminar.