Nonlinear　acoustic　coefficients　have　a　close　relationship　with　structural　cracks.　A　nonlinear　ultrasonic　modulation　method　for　micro-crack　quantitative　evaluation　is　developed.　The　influence　of　phase　threshold　on　the　crack　evaluation　is　discussed.　Ultrasonic　modulation　using　a　unilateral　incentive　model　is　applied　to　the　quantitative　evaluation　of　micro-cracks　for　different　specimens.　The　experimental　results　indicate　that　the　method　can　be　beneficially　applied　for　micro-cracks　detection.　A　proper　phase　threshold　can　improve　the　reliability　of　the　method　based　on　nonlinear　ultrasonic　modulation.　The　presented　modulation　factor　can　be　used　to　quantitatively　evaluate　the　structural　cracks,　and　it　is　suitable　for　small-crack　quantitative　evaluation.　(C)　2015　Elsevier　Ltd.　All　rights　reserved.