[en] The purpose of the assessment process of large thermal-hydraulic system codes is verifying their quality by comparing the code predictions against the experimental data. The Fast Fourier Transform Based Method (FFTBM) has been used widely for quantification of the prediction accuracy regardless of its limitation that it does not provide any time resolution for a local event. This limitation can be overcome by a wavelet transform because the resolution of the wavelet transform effectively varies in the time-frequency plane depending on the choice of basic functions. A wavelet transform is applied to an assessment of thermal-hydraulic system code with several parameter data of the ISP-50. To compare code prediction and experimental data, the cross-wavelet-transform and wavelet-coherence, which are methods to present similarity of two 1-D data, were also applied. Additional method that a pattern analysis using the cross-wavelet-transform and wavelet-coherence is conducted for a representative value to verify a similarity between the two data. (author)