The effect of glass-fiber epoxy interface in cross-ply reinforced composites on the fatigue behavior is studied by using load-increasing fatigue test. 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 on test conditions. Energy loss is shown to be a sensitive tool to characterize the nature of fiber matrix adhesion. The energy loss 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. (C) 2003 Kluwer Academic Publishers.