The porous structures and electrochemical double layer capacitance of activated carbon microbeads and carbon fibers were investigated using nitrogen gas adsorption and electrochemical constant current cycling (CCC) methods. Porous structural information on pore size distribution (PSD) and surface area were extracted through a gas adsorption analysis program based on density functional theory (DFT). The relation between the porous surface areas (ranging from 500 m(2)/g to 1700 m(2)/g) and specific double layer capacitance is discussed and a simple model is presented to correlate these parameters. It was found that the specific double layer capacitance on the micropore surface (about 15-20 mu F/cm(2)) was determined by the property of carbon basal layer. Additionally, the specific double layer capacitance on the external surface strongly depends on the surface morphology and possibly also on the functional groups. It was found that the magnitude could change significantly from one carbon type to another.