Based　on　elastic　mechanics　theory,　the　multi-modes　and　disperse　characteristics　of　guided　circumferential　waves　in　pipe　were　investigated　theoretically　and　experimentally,　the　disperse　curves　of　guided　circumferential　waves　were　gotten　by　numerical　calculations.　The　relationships　between　the　angle　of　beam　transducer,　frequency　and　guided　circumferential　modes　were　analyzed　by　our　guided　wave　experiment　system.　Then　single　guided　circumferential　mode　was　excited　in　a　pipe　(O.D　88.8　mm,　I.D　80.8　mm).　Then　an　artificial　longitudinal　defect　(25　×　1　×　0.7　mm3)　on　the　surface　of　a　pipe　was　detected　by　use　of　the　single　guided　circumferential　wave.　The　results　show　that　single　guided　circumferential　mode　can　be　excited　in　a　pipe　by　choosing　special　frequency　and　special　angle　beam　transducer,　as　the　same　as　the　case　of　exciting　Lamb　wave　in　a　plate,　and　it　can　be　used　to　find　the　longitudinal　defect　on　a　pipe　surface.