Fatigue behaviors of glass fiber-reinforced poly (butylene terephthalate) (PBT) were studied based on dynamic viscoelastic measurements during the fatigue process. The fatigue strength of glass fiber-reinforced PBT was greatly improved by strengthening the interfacial adhesion between glass fiber and matrix PBT. The heat generation rate under cyclic fatigue for PBT reinforced with surface-unmodified short glass fiber was always larger than that reinforced with surface-modified short glass fiber because of the large net imposed strain amplitude of PBT matrix which occurred due to the interfacial debonding under cyclic fatigue. A fatigue fracture criterion based on the magnitude of hysteresis energy loss being consumed for a structural change was established for the PBT/short glass fiber composites in consideration of glass fiber-matrix polymer interfacial interaction.