Microstructure and electric conductivity of PAN-based activated carbon fibers (PAN-ACF) were investigated using tension and KOH activation. The application of tension during stabilization decreased pore volume as well as specific surface area. Increase of KOH solution concentration caused serious damage to the surface of PAN-ACF. This surface damage of PAN-ACF resulted in increase of specific surface, pore volume and wider pore size distribution. PAN-ACF with higher tension showed higher electric conductivity. However, the electric conductivity was decreased by the increased BET surface area.