A previously proposed method for the reliability analysis of brittle structures subjected to thermal shocks is extended to include the uncertainties about the magnitudes of the applied thermal shocks and the fracture strength of the brittle materials. Flaws with a given probability distribution of size are assumed to be distributed at random with a certain density per unit volume of the composed material. A probabilistic failure model in which all kinds of uncertainties can be included is presented to study the probability of crack instability of a unit volume of brittle material. The reliabilities of brittle thermal systems are evaluated based on the weakest-link hypothesis and the proposed probabilistic failure model. A numerical example is given to demonstrate the application of the present method.