The U-type parallel channels for large size planar solid oxide fuel cell (pSOFC) are systematically optimized with the computational fluid dynamics (CFD) method. The CFD calculations are based on realistic 3D gas channel models and typical pSOFC working parameters. The optimized geometric parameters include the height of interconnect ribs, aspect ratios of the inlet-header and outlet-header cross-sections, the sum of inlet- and outlet-header widths and the ratio of the outlet-header width to the inlet-header width (a). Detailed CFD calculations show that a Suitable at and a relatively large header width are necessary for the flow uniformity of both air and fuel in large size pSOFCs. In particular, alpha is demonstrated to be a key parameter for the flow uniformity of large size pSOFCs with U-type parallel channel designs and a proper choice of at is of critical engineering importance. The physical origin for the importance of alpha on the flow distribution is analyzed. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.