To investigate the formation of grain boundary barriers, composite SnO(2)-Zn(2)SnO(4) varistor ceramics were prepared by traditional ceramic processing and, the electrical properties of grains and grain boundaries were evaluated by the measurement of complex impedances spectra. Calculated using the low frequency impedance data for each sample, a lower value about 0.09-0.32 eV and a higher value about 0.50-0.83 eV of activation energy E(a) was obtained in the temperature range of 50-110 and 140-200 degree C, respectively. The simulated results of complex impedances spectra by "Zsimpwin" software indicate that the equivalent circuit for SnO(2)-Zn(2)SnO(4) composite ceramics is connected in series by one RC and two QR parallels. The relaxations in the dielectric dispersion spectra at low frequency of ~1000 Hz suggest that one of the QR parallels is corresponding to oxygen vacancy. With increasing sintering temperature, the E(a) values related to oxygen vacancy decreased whereas, the values related to grain boundary barriers increase obviously. The results suggest that the oxygen vacancy is the key factor to the semi-conductance of SnO(2) grains and formation of grain boundary barriers.