A fracture-mechanical theory for thermal stress fracture analysis of brittle bodies subjected to thermal shocks is presented. Criteria of crack instability for brittle solids under general thermal stresses are derived. Flaws in the form of circular Griffith microcracks are assumed to be distributed at random with a certain density per unit volume in the bodies. A body fails when the effective thermal stress at any differential volume reaches a critical value for crack instability. The critical temperature difference required for the failure of the body can be determined by the proposed method. Examples on the evaluation of the critical temperature differences of several brittle bodies are given to demonstrate the applications of the proposed method.