An electrochemical method was applied to determine the boundary between stagnant and mixed regions of a plastic fluid or yield stress fluid. Transparent solutions of carboxybinyl polymer were used as the test fluids, whose viscosities were increased by adding NaOH. The electrolysis of water was adopted as the electrode reaction, since the addition of metal ions to the solution caused a large decrease in viscosity and yield stress. A handmade probe of a tiny Pt wire of I mm. length was used as the cathode, which was fitted to a rotating arm and moved at a constant velocity in a stagnant liquid in order to measure the dependency of probe velocity on the current-voltage curve. Measurements were carried out by applying a potential of 1V between the electrodes, which is the maximum voltage applicable to the electrode without generation of hydrogen bubbles. The sensitivity of the current to the probe velocity increased with decreasing velocity, which shows the availability of this method for determining the boundary of stagnant and flow regions. The method was applied to determine the boundary of the flow region or the cavern formed near a rotating Rushton turbine impeller. The probe was traversed on the r-z plane in the vessel to measure the current through the probe, and the boundary was determined as the position at which the distribution of the current varied sharply. The cavern boundary determined in this manner was located outside of the cavern boundary that was visualized by injecting Congo Red into the impeller region. This indicates that the flow outside the visualized cavern is tangential. It was found that the current fluctuation consists of two components: the higher frequency coincides with the blade passing frequency, and the lower one represents the frequency of expansion and contraction of the cavern.