Toward　invalidity　of　the　linear　ultrasonic　technique　for　nondestructive　detection　of　micro-crack　in　structures,　a　frequency-mixing　nonlinear　ultrasonic　technique　was　developed　for　micro-crack　detecting.　Firstly,　the　mechanism　of　frequency-mixing　and　signal　processing　method　for　feature　extraction　were　investigated;　secondly,　By　experimental　measurement　of　frequency　response　of　mixing　frequencies,　the　optimum　parameters　for　frequency-fixing　detection　frequency-mixing　were　determined;　experiments　for　micro-crack　detection　were　conducted　in　mode　of　heterolateral　excitation,　and　research　focused　on　the　influence　of　frequencies　of　excitations　on　the　detection　results.　It　was　shown　that　in　mode　of　heterolateral　excitation,　not　only　fatigue　micro-cracks　in　the　structure　can　be　detected,　but　also　the　position　of　the　defect　can　be　located.　Meanwhile,　the　frequencies　of　excitations　have　great　influence　on　signal-to-noise　ratio.　The　performance　of　damage　detection　experiments　was　best　when　the　amplitudes　of　mixing　frequencies　are　maximum.　Therefore,　frequency-mixing　nonlinear　ultrasonic　technique　is　a　valid　approach　for　crack　detection　and　location.