Background: The uncertainties of input parameters in an deterministic structural integrity assessment of pressure vessels may affect the assessment results.This can be improved by performing probability facture mechanics（PFM） analysis.Purpose: This work investigates the effect of uncertainties of load

defect size

fracture toughness and failure criteria on the failure probability of semi-elliptical surface cracks in plates under combined tension and bending.Methods: The correction factor method provided by EPRI is used to estimate the stress intensity factor（SIF）.The J-integral values at the deepest point of the surface crack tip are evaluated using the reference stress method and the globe limit load solution developed by Goodall & Webster and Lei.PFM analysis is performed with considering the uncertainty of crack size

yield strength and fracture toughness and Monte-Carlo（MC） simulation is used to calculate the failure probability.Results: Failure probability increases with increase of load level

Lr

for all load ratio values considered in this work for a given failure criterion.However

the failure probability based on the elastic-plastic fracture criterion is higher than that based on the linear elastic fracture criterion for a given load lever

Lr.Conclusions: The load level and the failure criteria have significant effect on the failure probability.However

the load ratio makes a little contribution to the failure probability for a given failure criterion.