A method for the reliability analysis of brittle structures subjected to thermal shocks is presented. Flaws with a given probability distribution of size are assumed to be distributed at random with a certain density per unit volume in the structures. Criteria for crack instability are derived for brittle solids under general thermal stresses. A probabilistic failure model is presented to study the probability of crack instability for brittle solids containing cracks with uncertain crack size. The reliabilities of the brittle structures are evaluated based on the weakest-link hypothesis, which states that a structure fails when the cracks in any differential volume become unstable. A numerical example is given to demonstrate the application of the proposed method.