Abstract: This paper presents the mechanism of the breakdown of liquids. Experimental apparatus of pulsed breakdown of the pressured deionized water has been set up. The relationship between the pulsed breakdown stress and the water pressure has been investigated. The results of test show that breakdown stress is significantly higher than that one might predict by currently accepted breakdown criteria when a hydrostatic pressure is applied to the water dielectrics. When the pressure of water changed from 0.1MPa to 0.7MPa, breakdown stress of water changed from 25.3MV/m to 46.7MV/m. Using the pressured deionized water as dielectrics, the energy density of the energy system could be enhanced notably and the size of the pulsed power system could be minified too.