The effect of glass-fiber epoxy interface in cross-ply reinforced composites on the fatigue behavior is studied throughout this: paper. The sensitivity of the interphase on fatigue life of cross-ply laminates is shown by a shift of the S-N curve to approx. 30% higher applied maximum loads for the composites with well-bonded fibers. Further, the damage as measured by stiffness reduction is more significant for the composites with poor bonded and commercial sized fibers than was found for EP sized ones. The loss energy is shown to be a sensitive tool to characterize the nature of fiber-matrix adhesion. The loss energy for composites with poor adhesion between fiber and matrix results in significantly higher amounts of consumed energy during a single stress-strain than those composites containing well-bonded fibers.