After the production of a typical vessel used to store test，many cracks in the welding seam have been observed. According to probabilistic fracture mechanics，the reliability of the pressure vessel is given by R=1-∫^+∞_-∞?(a)da∫^a₀Φ(a_c)da_c Our analysis of data from 165 samples has shown that the fracture toughness K_cobeys the normal distribution and the second type of exponential distribution for the crack sizes is better applicable to fit the data of crack sizes in a welding seam. The reliability is calculated by using Monte Carlo method.