A deterministic fracture mechanics analysis does not address the uncertainties involved in material properties, magnitudes of loads, location and size of the flaws, etc. However, in a real life situations such uncertainties can affect significantly the conclusions drawn out of a deterministic analysis. The principles of probabilistic fracture mechanics may be used to ascertain the effects of such uncertainties. A computer code PARISH (Probabilistic Assessment of Reactor Integrity under pressurised thermal SHock) has been developed based on principles of PFM for analysing a reactor vessel subjected to pressurised thermal shock. The code assumes a crack in the reactor vessel of random dimension depending upon Marshall flaw depth cumulative distribution function. The applied SIF at the tip of this crack is computed either using closed form solution or a precomputed data base. The material K IC is then calculated using the crack tip temperature and RT NDT. The value of RT NDT depends on the initial value of RT NDT and the increase in the value of RT NDT depending upon the fluence, copper content and nickel content. A Gaussian distribution is assumed for these parameters. If the applied SIF is more than the material K IC, the crack is assumed to propagate. The crack can be arrested only if the applied SIF is less than the material K Ia at that location. The material K Ia again depends upon the RT NDT, which in turn depends upon the fluence, copper content and nickel content of the material at that location. The vessel failure is assumed if the crack propagates by the 75% of the thickness. Such procedure is repeated for large number of cracks (of the order of one million). Using Monte-Carlo simulation, probabilities of no crack initiation, crack initiation and vessel failure are calculated. The present probabilities are conditional in the sense that the transient is assumed to occur. The case studies are presented involving a nuclear reactor vessel subjected to two different kinds of pressurised thermal shocks.