Multiferroic BiFeOδ ceramics of various oxygen contents (δ) were prepared by solid state reaction. Frequency dependence of dielectric properties of BiFeOδ samples at room temperature was measured in the frequency range from 40Hz to 1MHz with a HP4294A precision impedance analyzer. For all the samples, the dielectric constant and loss decrease with measurement frequency. The effect of introducing oxygen vacancy or excess oxygen is to reduce the dielectric constant of BiFeO3. The dielectric loss in BiFeOδ increases with oxygen content. The large change (10%—35%) in the dielectric properties of BiFeO3 samples with various oxygen content(δ) may be understood in terms of the space charge limited conduction associated with crystal defects. The effects of oxygen content on the crystal structure of BiFeO3　ceramics were also investigated using positron annihilation technique (PAT). It has been found that the mean positron lifetime in BiFeOδ　is strongly dependent on the oxygen content. Introducing oxygen vacancy leads to an increase in positron lifetime and a decrease in local electron density of BiFeO3 ceramics.