In　engineering　practice,　failures　due　to　fatigue　cracks　in　metallic　structures　have　always　been　difficult　to　predict.　In　our　study,　nonlinear　Lamb　wave-mixing　was　applied　in　the　detection　of　micro-cracks　in　plates.　The　analysis　of　the　nonlinear　interaction　of　these　waves　with　cracks　of　various　lengths　and　widths　was　performed　using　finite　element　simulations.　The　simulation　results　showed　that　the　sideband　at　the　sum　frequency　provide　a　sensitive　means　for　micro-crack　detection.　Moreover,　the　sideband　amplitudes　show　a　monotonic　increase　with　micro　crack　length,　but　a　decrease　with　micro-crack　width.　Experiments　using　Lamb　wave-mixing　were　conducted　on　plates　with　fatigue　cracks.　The　experimental　results　show　that　a　proposed　acoustic　nonlinearity　parameter　related　to　the　sideband　at　the　sum　frequency　is　sensitive　to　micro-cracks　in　plate,　and　is　well　correlated　with　the　damage　degree.