In this paper fluid dynamic behaviors of cathode gas flow in an external gas manifold of a planar solid oxide fuel cell (SOFC) stack are simulated to investigate the overall pressure variation and flow distribution. External manifold models are built in three dimensions for a 60-cell planar SOFC stack. Cell units of the stack are treated as porous media with appropriate resistances, which were determined by the previous results of cell modeling. In order to simplify this model, electrochemical reactions, heat and mass transport phenomena are ignored inside cells. The flows of cathode gas in the external manifolds of stack are modeled by means of computational fluid dynamics (CFD) methods. A commercial CFD package “Fluent” was used for geometry creation, grids generation of flow volume interiors, solving mass, momentum equations, plotting computational results. The detailed results of pressure variation and flow distribution of gases in the stack were achieved. The effects of different designs and parameters such as a gas distributor inside the external manifold, the permeability of porous media in cells and cathode gas feeding rate on gas distribution and pressure variation are studied. Comparison of different cases is carried out by the modeling results. Modeling results show for the proposed stack design in this paper the additional gas distributor located in the center of the inlet manifold and a rise of resistance in cells can respectively enhance the uniformity of flow distribution over 60 cells.