In this study, viscose rayon-based activated fabrics were used as the electrodes of supercapacitors. First, viscose rayon knitted fabrics underwent oxidation, carbonization and activation in a semi-open high-temperature erect furnace to produce activated carbon fabrics (ACFs). They were then treated at temperatures up to $1500^{circ}C$ for one hour. Electrochemical properties of ACFs were investigated by cycle voltammetry and electrochemical impedance spectroscopy. The ACFs after high temperature treatment has improved conductivity and substantially increased mesopore ratio, yielding higher capacitor retention in rapid charging-discharging processes. It is shown that the ACFs treated at $1500^{circ}C$ had the highest mesopore ratio of 83 %, specific surface area of $1254m^2/g$ and average pore diameter of $20.9{AA}$, resulting in lower resistance of $0.2{Omega}-cm$. In addition, this ACFs electrode showed the highest capacitance retention of 49 % at high charging speed of 250 mV/s.