Abstract: The single phase La1.5Sr0.5NiO4-δ ceramic was prepared by solid sintering method at 1 360 ℃ in air and then annealed at 800 ℃ for 3 h under vacuum of 2×10-3 Pa. The AC impedance spectrum for variable temperature of the ceramic before and after vacuum annealing was analyzed by impedance analyzer. The results show that the large dielectric constant before vacuum annealing was about 105 under the conditions of 100-320 K and 102-106 Hz and the large dielectric constant originated from charge-ordering. The dielectric constant reduced to 104 after annealing, meanwhile, the diffuse area shifted to low frequency radically, and the polarization mainly came from grain boundary polarization.