Abstract: Based on the analysis of the process and the phenomena during a large break loss of coolant accident (LBLOCA), three models utilized in RELAP5/MOD3.3 (CHF model, film boiling heat transfer model and critical flow model) were identified to have a great impact on the peak cladding temperature. Through the literature review, the corresponding experiment data of these three models were obtained. Meanwhile, based on the comparison between the experiment data and the calculation results of the models, the uncertainty ranges and probability distributions of these three models were obtained by applying the orthogonal series estimation method. The uncertainty analysis of LOFT LP-02-6 LBLOCA experiment by applying probability assessment results shows that the uncertainty ranges of the RELAP5 heat transfer models are reasonable.