The effects of Bi-substitution on the complex impedance spectra of yttrium iron garnet (YIG) were studied in this paper. The polycrystalline yttrium iron garnet Y2BiFe5O12 (YIG: Bi) and Y3Fe5O12 (YIG) samples were prepared by solid-reaction method. The complex impedance spectra were measured in the frequency range from 1 KHz to 100MHz at several temperatures between 210oC and 500oC. The complex impedance sample shows that the YIG: Bi can be represented by double Cole-Cole semicircles, and the YIG can be represented by a single Cole semicircle. The physical structure of the specimen was visualized as comprising of small resistance grains separated by large resistance grain boundaries in accordance with the impedance spectra observations. The electrical processes in the sample were modeled in the form of an electrical equivalent circuit made up of a series combination of two parallel RC circuits attributed to grains and grain boundaries. The temperature dependence of bulk resistance indicated an evidence of Arrhenius-type thermally activated process, showing a close to linear variation up to a temperature of 740 K.