The dielectric characteristics of Barium titanate (BT) based ceramics are evaluated in a broad frequency range at room temperature and at elevated temperature; the measured frequency is 0.1Hz to 1GHz. The results showed that the dielectric constant increases sharply with the decreasing frequency and is higher than 3000 appeared in lower frequency for Mg 1% doped BT bulk ceramics, which may be resulted from space charge polarization becoming more dominant as compared to the dipolar polarization especially when the frequency is lower than 1 kHz. The dielectric relaxation peaks of Mg-doped BT bulk ceramics appeared both at low and radio frequency whereas pure BT ceramics showed relaxation peak only at radio frequency, what's more, the relaxation frequency of Mg-doped sample is rather low than pure barium titanate ceramics. The dielectric relaxation motion approximately obeys Debye relaxation model approved by Cole-Cole diagram. Thermal activation energy is 0.46eV obtained by Volgel-Fulcher law fitting, which demonstrated that dielectric relaxation due to oxygen vacancies in the Mg-doped BT ferroelectric ceramics.