Abstract In this paper a probabilistic Leak-Before-Break analysis for WWER-1000 unit was performed based on the Failure Assessment Diagram (FAD), treating strength and crack morphology parameters as stochastic values. In order to perform probabilistic calculations Critical temperature of brittleness and Yield (Ultimate) Stress, were fitted by normal distribution, and based on experimental data taken from the manufacture documentation found at the Ukrainian NPP. The statistical behavior of the leak rate (LR) and Critical crack length was examined. The failure probability was calculated using Monte-Carlo simulation, for different types of cracks. It was proven, then crack morphology parameters highly affects the leak rate, among the mechanical characteristic a Fracture toughness has more influence rather than Ultimate of Yield strength.