In this paper, we reveal the complex permittivity properties of fresh water under a high electric field. First, the measuring instruments of parallel plate capacitor and needle-plate electrode are designed to apply a high electric field to freshwater. Then, we measure the changes in complex permittivity to maximum RF power of 1 kW. Each instrument calculates the electric field intensity at 1 kW by using electromagnetic field analysis. From our above results, we clarify that the complex permittivity of fresh water at 13.56 MHz does not change under 9.7 MV/m or less.