The effect of glass-fiber epoxy interface in cross-ply reinforced composites on the fatigue behavior by using load-increasing fatigue test is studied throughout this paper. The damage as measured by stiffness reduction is more significant for the composites with poor bonded fibers as was found for EP sized ones, dependent from test conditions. 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 loop than those composites containing well-bonded fibers.