Fe2+ in K1.22Fe11O17 single crystals has been oxidized by iodine to obtain an ideal composition of KFe11O17. The crystal structure was analysed based on its beta-alumina structure and a final R(F) value of 0.034 was obtained. The atom positions in the spinel block were unchanged by the oxidation. On the other hand, while the amount of K+ in BR sites was comparable to that in K1.22Fe11O17, that in the aBR sites was largely decreased. The ionic conductivity of beta-ferrite with the ideal composition was measured at 300 degrees C and found to be 3.53 . 10(-3) Omega(-1) cm(-1) in the (001) plane. The activation energy was E(a) = 0.19 eV. This ionic conductivity was twenty times lower than that of K1.22Fe11O17. The activation energy was also lowered. This is because the amount of K+ in aBR sites was relatively small. The electronic conductivity was also measured and found to be 2.05 . 10(-5) Omega(-1) cm(-1) at 300 degrees C, which is two orders of magnitude smaller than that of K1.22Fe11O17. However, the activation energy (0.45 eV) was unchanged. The hopping pairs of Fe2+ and Fe3+ would be largely decreased by the oxidation.