The fatigue behavior of segmented polyurethane and the effect of aging following fatigue were investigated by a combination of thermoluminescence (TL) and pulse NMR. It was found that the peak at 405 K, related to the segmental motion in the interfaces around the hard domains, decreased in intensity in the early stage of fatigue and disappeared at 1.0 × 105cycles. Meanwhile a new TL peak near 390 K (peak I) appeared and increased in intensity sharply during fatigue up to 1.0 × 105cycles. In this process the peak at 440 K (peak H) decreased in intensity correspondingly. The maximum temperatures of both peaks remained almost unchanged during fatigue. On the other hand, in the aging process following fatigue, this newly appeared TL peak decreased in intensity and the higher TL peak regained intensity accordingly. The TL and NMR results showed that the enhanced lower TL peak during fatigue is related to a fatigue-induced phase-mixed phase, which has a segmental mobility higher than the interfaces around the hard segment domains, and that this phase-mixed intermediate phase increases in volume fraction as the fatigue proceeds but decreases and is transferred partly to the hard segment domains during aging following fatigue.