The present study describes a design on geometrical parameters of honeycomb membrane structure for the AMTEC cell. This concept uses small channel honeycomb separator made of beta″-alumina solid electrolyte. The dimension of the channel must be decreased as small as possible to increase the electrode area contained in unit volume of the cell. On the other hand, the narrow path for sodium vapor flow significantly increases the pressure drop and decreases the cell performance. A numerical calculation approach to analyze heat and mass transfer phenomena inside the cell was introduced. The effect of geometrical parameters and operating parameters was evaluated The calculated results show that the honeycomb membrane of 2 mm in pore side length operating at 1000 K has a predicted output power density of 50 kW per square meter of its condenser area. This performance is five times as high as hat of the conventional type cell. The conversion efficiency and the influence of temperature drop are also discussed.