Stabilized Bi2O3 has gained a considerable amount of attention as a solid electrolyte material for low-temperature solid oxide fuel cells due to their superior oxygen-ion conductivity at the temperature of relevance (≦ 500°C). Despite many research efforts to measure the transport properties of stabilized Bi2O3, however, the effects of grain boundaries on the electrical conductivity have rarely been reported, and their results are even controversial. Here, we attempt quantitatively to assess the grain boundary contribution out of the total ionic conductivity at elevated temperatures (350-500°C) by fabricating epitaxial and nano-polycrystalline thin films of yttrium-stabilized Bi2O3. Surprisingly, both epitaxial and polycrystalline films show nearly identical levels of ionic conductivity, as measured by AC impedance spectroscopy, and this is the case despite the fact that the poly-film possesses nanosized columnar grains and thus an extremely high density of the grain boundaries. The highly conductive nature of grain boundaries in stabilized Bi2O3 is discussed in terms of the clean and chemically uniform grain boundary without segregates and the implications for device application are suggested.