The　non-collinear　wave　mixing　technique　was　used　for　the　quantitative　evaluation　of　fatigue　cracks.　Two　shear　waves　intersect　and　interact　with　fatigue　cracks,　which　lead　to　the　generation　of　a　longitudinal　wave.　Non-collinear　wave　mixing　experiments　were　conducted　on　different　specimens,　whereby　the　position　at　which　the　two　incident　waves　intersected　was　scanned　vertically　below　the　notch　and　the　measured　signals　were　processed　by　filtering,time-frequency　analysis　and　bispectral　analysis.　The　presence　of　a　wave　packet　in　waveform,　or　wave-mixing　component　at　the　sum　frequency　in　time-frequency　domain,　or　bispectral　peak　at　the　sum　frequency　was　used　to　characterize　the　fatigue　cracks.　The　length　of　the　fatigue　cracks　in　two　samples　was　estimated　according　to　the　spatial　distribution　of　the　ultrasonic　nonlinear　coefficient.　The　measured　results　corresponded　closely　with　the　actual　lengths;　therefore,　the　proposed　method　is　effective　for　the　quantitative　evaluation　of　fatigue　cracks.