Abstract: 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.