Abstract: 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.