Abstract: Internal flooding events of nuclear power plants are a kind of common cause failure which may cause multi-component failure simultaneity. Internal flooding analysis, using probabilistic safety assessment, can find out the weakness of the plant and improve its design. Based on a 300 MWe nuclear power plant, the method of internal flooding level 1 probabilistic safety assessment was introduced and the core damage frequency due to internal flooding was calculated with RiskSpectrum code. The calculation results show that internal flooding induced CDF contributes 2.45% of the total internal events CDF which is a small fraction.