[en] The report describes the theoretical study and the experiment of ultrasonic guided waves which are attractive for the online and real time inspection of pipes installed in nuclear power plants. The following list presents the main contents of the report. The characteristics of guided wave, mode, dispersion curves, and Short Time Fourier Transform(STFT) which is necessary for the analysis of guided wave signal were studied and summarized. The dispersion curves of steam generator tube was calculated. The incidence angle of ultrasonic wave to generate the specific mode was also calculated. And the modes were identified through the measurements of group velocity and STFT. We also identified the mode conversion of guided wave when it passed through the bent part of the tube. We integrated FR signal, Fourier transform data, phase velocity dispersion curve, group velocity dispersion curve, and STFT into one comprehensive method that could identity and analyze the complicate modes of guided waves. We prepared the criteria which are necessary to the selection of the optimal mode for the detection of flaws, and selected the most suitable mode in the detection of flaws. Finally we validated it through the experiments