In order to improve sizing accuracy of a pipe discontinuity using ultrasonic guided waves, an analysis method to obtain the reflection coefficients of discontinuity is proposed and the discontinuity-length sizing potential by sweeping frequency is discussed. In this method, reflection coefficients are calculated by integration of reflection waves from divided calculation regions having different characteristic acoustical impedances.The reflection coefficients were analyzed for several shapes of discontinuities on 2-D analysis models, and the reflection coefficients changed cyclically depended on frequency. The effectiveness of the method was confirmed experimentally using torsional mode guided waves (about 40 kHz) that were propagated in pipes with diameters of 60 mm or 114 mm and in which discontinuities (ratio of depth to length = 1:8, 1:16, 1:32) were fabricated.By the results, the trends of analysis results were generally in good agreement with that of experimental results, and the discontinuity-length can be estimated by the relationship between frequency and amplitude, in case that the shape of discontinuity is pre-estimated or assumed.