Based　on　the　theory　of　nonlinear　ultrasonic,　material　early　mechanical　degradation　is　associated　with　the　microstructural　changes　such　as　dislocation　and　slip　band.　When　a　single　sinusoidal　ultrasonic　wave　is　launched　to　a　nonlinear　medium,　the　higher　harmonics　will　be　generated.　A　FEM　model　of　nonlinear　was　established　by　a　special　element　which　account　for　a　nonlinear　stress-strain　relation.　Calculation　was　performed　for　the　influence　of　the　micro-defect　with　different　length　and　quantity　to　ultrasonic　nonlinearity　parameter.　This　research　develops　a　robust　experimental　procedure.　There　is　linear　relationship　between　amplitudes　of　the　second-harmonic　waves　and　the　squared　fundamental　waves　as　a　function　of　increasing　input　voltage　amplitude.　Using　this　system,　ultrasonic　nonlinearity　parameters　of　a　magnesium　sample　were　measured.　The　experimental　results　show　that　there　is　a　significant　increase　in　nonlinearity　parameters　linked　to　fatigue　degree;　Ultrasonic　nonlinearity　parameters　can　characterize　the　early　fatigue　damage　of　magnesium.　©2010　IEEE.