Na0.25K0.25Bi0.5TiO3(NKBT50)lead-free ceramics were prepared by solid-state reaction. The dielectric property of the ceramics was investigated in the lemperature range from room temperature to 400℃. Our results show that there exists a dielectric anomaly in the vicinity of 70℃ for the unpoled ceramics sintered in air, which will disappear after the ceramic were poled or sintered in oxygen. It is believed that the dielectric anomaly is caused by defect dipoles related to oxygen vacancies. Thermal stimulated depolarization current shows that the materials are depolarized at 225℃ and the dielectric anomaly corresponding to this phase transition is also affected by oxygen vacancies.