A computational simulation was conducted by using a one-dimensional isothermal model for an alkaline fuel cell (AFC) single cell to investigate influences of the thicknesses of the separator, catalyst layer, and gas-diffusion layer in an AFC. The cell polarizations were predicted at various thicknesses and their influences were also analyzed. Thickening the separator layer decreased the limiting current density and increased the slope of the ohmic polarization region. Investigation on the thickness of the anode catalyst layer showed that the optimum thickness varied between 0.04-0.15 mm according to the cell voltage. The thickness of the cathode catalyst layer significantly influenced the cell performance. Also, a limitation of thickness effect in the cathode catalyst layer was observed. This limitation was considered to be caused by the mass transfer resistance of the electrolyte.