For　the　poor　sensitivity　of　traditional　ultrasonic　technique　for　micro　crack　detection,　a　kind　of　nonlinear　ultrasonic　technique,　vibro-acoustic　modulation　technique,　is　used　for　micro　crack　detection　in　pipes.　The　influence　of　the　frequency　and　amplitude　of　vibration　on　the　modulation　index　(MI)　is　experimentally　investigated.　The　experiment　results　prove　that　the　vibro-acoustic　modulation　technique　can　be　used　for　crack　detection　in　pipes.　The　modulation　index　is　influenced　by　the　frequency　and　amplitude　of　vibration,　therefore　it　is　important　to　choose　the　appropriate　parameters　in　order　to　get　higher　sensitivity　in　crack　detection.