For planar SOFCs the seal is a critical component, potential fracture in the seal needs to be investigated in order to enhance the reliability or the seal, A model based on the classical beam bending theory and the fracture theory of ceramic materials has been developed for predicting the crack extension in the seal. The model reveals that the resistance of the seal to cracking on cooling is mainly affected by two factors: the seal thickness and the CTE mismatch. Furthermore, a cracking diagram is established to reveal the effects of the seal thickness and CTE mismatch on the crack extension behavior. It shows that the 'no cracking' area increases with decreasing seal thickness, and larger CTE mismatch requires a thinner seal to avoid cracking. The model and the cracking diagram are experimentally validated through monitoring the leakage rate of a glass-sealed chamber. and the crack extension deduced from the measured leakage rate shows good agreement with those predicted by the model. The proposed model can serve as a useful tool in sealing design of SOFC. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.