The discharge and charge performance of polyaniline which has a fibrous morphology was investigated from microscopic current density and anion diffusion time constant measurements. The microscopic discharge current density is calculated from the radius of the polyaniline fibril, assuming the true electrode surface area corresponds to the surface area of the long polyaniline fibril (2 alpha L, L:the sum of the length of the polyaniline fibril). The time constant for the diffusion of anions in the polyaniline fibril was calculated from alpha(2)/D, where D is the diffusion coefficient and a is the polyaniline fibril radius. The diffusion coefficient was measured using an impedance method, and the alpha(2)/D parameter was calculated. Galvanostatic discharge and charge cycle tests were carried out to decide the performance of polyaniline cathodes with different fibril radii. From these experiments, it can be seen that the dependence of the discharge characteristics on the fibril radius is diminished by the positive dependencies of the diffusion coefficient and roughness of the polyaniline fibril on the fibril radius.