The influence of R2O3 substitution on the electrical properties and the thermoelectric power on the W-type hexaferrite Ba0.95R0.05 Mg-0.5 Zn-0.5 CO Fe-16 O-27; R=Y3+, Er3+, Ho3+, Sm3+, Nd3+, Gd3+, and Ce3+ ions as a function of the absolute temperature and frequency in the ranges 300-750 K and 100 kHz-4 MHz, respectively have been studied. The results obtained reveal that, by introducing a relatively small amount of R2O3 instead of Fe2O3, an important modification of both structure and physical properties can be obtained. The dielectric constant epsilon', the dielectric loss factor epsilon '' and AC conductivity sigma of the investigating samples have highest values in the case of Sm3+. The values of the calculated activation energy varied between (0.335 and 0.553 eV) and (0.106 and 0.232 eV), above and below the transition temperature, respectively which indicate the semiconducting properties of the prepared samples. The conduction mechanism for Ho3+ sample is associated with the small polaron whereas the hopping conduction mechanism is predominant mechanism for the samples doped with the rest of rare-earth ions. The thermoelectric power measurements indicated that the samples are n-type semiconductors except Er3+ and Y3+ samples, where the thermoelectric power of them is positive at low temperature but nearly above 480 and 375 K, respectively transits to negative. The charge carriers concentration n increases with slightly increasing RE ionic radius to critical value (approximate to 1.04 angstrom), after that n becomes nearly constant. (C) 2008 Elsevier B.V. All rights reserved.