A new method of investigation for high voltage processes in liquid dielectrics by means of non linear optical effect is established. Non linear optical effect suggests warm defocusing of laser beam, passing through the area of liquid dielectric, which is located in strong electrical field. Two cases are described in the paper: in first case electrohydrodynamic currents are defining processes, which appear in liquid under high voltage. In the second case, the change of the defocusing level of the laser beam is a consequence of structuring-by-the-space-charge of ions. Ions are forming cloud of electrical space charge. Described method has high sensitivity and allows to identify the edge of EHD-currents formation.