A straight capillary model is developed to estimate the mass leak rate of the cast ceramic tape seals for planar solid oxide fuel cells (SOFCs), which is further rectified with consideration of microstructure complexity including the tortuosity, cross-section variation and cross-link of leak paths. The size distribution of the leak path, effective porosity and the microstructure complexity are the main factors that influence the leak rate of the cast tape seals. According to the model, Al2O3 powders are selected for preparation of the seals by tape casting, and the leak rate is evaluated under various compressive stresses and gauge pressures. The results indicate that Al2O3 Powder with D-50 value about 2 mu m and specific surface area near 5 m(2) g(-1) can be used for the cast tape seals; and the obtained leak rate can satisfy the allowable leak limit. (C) 2009 Elsevier B.V. All rights reserved.