Considering　mechanical　properties　of　materials　for　early　material　degradation,　the　nonlinear　ultrasonic　characteristics　and　microstructure　changes　for　fatigued　LY12　aluminum　alloy　specimens　are　experimentally　investigated.　Results　show　that　the　degradation　at　early　stages　accompanies　by　the　ultrasonic　nonlinear　coefficient　changes　and　micro　morphological　variety.　The　method　combined　with　ultrasonic　nonlinear　measurements　and　microstructure　observation　has　a　potential　application　prospect　for　fatigue　life　assessment　of　early　degradation.