Abstract: In the best estimate plus uncertainty analysis of large break loss of coolant accident (LBLOCA), the key is the study on uncertainty analysis of reflood post-CHF heat transfer model. In this paper, the uncertainty of RELAP5 reflood post-CHF heat transfer model was studied. The transition boiling model of reflood was evaluated through the transition boiling test data at 0.1-0.4 MPa by Weisman and the film boiling model of reflood was evaluated through the film boiling test data at low pressure by Idaho National Engineering Laboratory (INEL). The probability distribution function of the model was obtained, which can be used in the LBLOCA uncertainty analysis.