It is very important to measure the electric field-strength in an insulator to understand a mechanism of dielectric breakdown. Recently, the development of the optoelectronics and the computer has been helped the measurement of electric field-strength in a liquid dielectric by optical method using the Kerr effect The sample liquid was nitrobenzene which has the Kerr constant of 3.62×10-12m/V2. Two test cells were used one was a brass coaxial cylindrical cell with two glass windows for the measurement of two-dimensional electric field vector and the other was a Teflon rectangular cell for the measurement of three-dimensional electric field vector. The He-Ne laser was used for a light source. The negative high voltage pulse was applied to the electrodes which were displaced in nitrobenzene. The intensity of the light transmitted through out the analyzer was measured with a photodiode. The electric field vector was calculated from the intensity of the measured light by the computer.