Aiming　at　the　detection　of　micro　crack　in　structures,　a　non-collinear　mixed　frequency　ultrasonic　method　for　detection　of　closed　cracks　was　developed.　Based　on　theoretical　analysis,　non-collinear　mixed　frequency　ultrasonic　testing　experiment　was　conducted　on　closed　cracks　in　structure.　Testing　signals　in　both　intact　and　cracked　specimens　were　filtered　and　analyzed　by　time-frequency　method.　Results　showed　that　the　closed　crack　can　be　recognized　by　the　emergence　of　obvious　wave　form　in　time-domain　filtered　signal,　or　the　sum-frequency　component　in　frequency　domain.　By　changing　locations　of　excitation　probes,　the　confluence　position　of　two　incident　waves　can　be　controlled　and　non-collinear　mixed　frequency　ultrasonic　testing　at　different　depths　was　achieved.　Moreover,　according　to　the　distribution　of　mixed　frequency　nonlinear　parameter　the　length　of　closed　crack　along　depth　was　measured.　The　research　makes　some　useful　efforts　on　quantitative　evaluation　of　micro-crack.　©　2017　Acta　Acustica.