Hot and cold fluids are mixed at the core outlet of sodium cooled fast reactors. The temperature fluctuation causes high cycle thermal fatigue in structural components. The temperature fluctuation at the core outlet region has not a sinusoidal waveform but an irregular spike waveform. In the study, the evaluation method for the high cycle thermal fatigue was investigated against the spike waveform of temperature fluctuation. It was found that the combinational method between the peak-to-peak value of temperature fluctuation and the frequency response function of structure could evaluate the fatigue damage in structure.