Background: Many investigations on the structural integrity assessment of reactor pressure vessel（RPV） under the pressure thermal stress（PTS） conditions have been performed.Probabilistic fracture mechanics（PFM） technique has been used for RPV PTS analysis since 1980.However

research on the structure integrity assessment techniques using the PFM method is insufficient in China to support the application of the method to be used for assessment of RPV under PTS conditions.Purpose: To gain the effective right assessment approach applied to Chinese RPV against PTS.Methods: Thermal stress distribution along the cylinder wall-thickness under PTS is firstly evaluated by FE in order to get the corresponding stress intensity factors（SIF） and to validate them with some available SIF solutions.The PFM analyses based on crack initiation are then performed under conditions of the crack size

fracture toughness and material yield stress.Both linear elastic fracture mechanics and R6 method are used in the analyses

and the sensitivity of uncertainties as well as the effect of the assessment method is discussed.Monte Carlo simulation is used to calculate the failure probability.Results: preliminary software is developed to perform the assessment for PTS analysis.The failure probability with R6 method is a little higher than with the linear elastic fracture mechanics approach.The effect of reference nil-ductility transition temperature RTNDT on the failure probability is more significant than other factors.Conclusions: Many uncertainties should be considered in RPV PTS analysis with PFM approach in the further

including the input parameters and failure models.Low RTNDT can extend the time of failure

and reduce the failure probability.