Abstract: A new CaO-TiO2-Al2O3 based dielectric ceramic system is designed for pulse forming line application. The dielectric ceramic is prepared by traditional solid state method and its microstructure was controlled by optimizing the composition and preparation process. The dielectric ceramic exhibits an excellent dielectric properties. The frequency stability of the dielectric constant is good. The dielectric loss is less than 0.002 in a wide frequency range. The electric strength is up to 50 kV/mm (1 mm in thickness). The influence of thickness on the electric strength of CaO-TiO2-Al2O3 based dielectric ceramics is studied. With the thickness decreasing from 1 mm to 0.1 mm, the dielectric breakdown strength increases nonlinearly from 50 kV/mm to 92 kV/mm. So it can be seen that the dielectric breakdown of the CaO-TiO2-Al2O3 dielectric ceramics is similar to their mechanical damage. Based on the chemical compositions and the microstructures, the dielectric breakdown mechanism of CaO-TiO2-Al2O3 dielectric ceramics can be explained by the weak breakdown theory.