Abstract: When the test temperature of superalloy materials is above 400 ℃, the visual method based on optical principle cannot be used to measure the fatigue crack length because of oxidation or high-temperature thermal radiation, so the direct current (DC) potential method needs to be used to measure the crack length. This paper introduces the process of measuring the crack length of compact tensile specimens and central tensile specimens by the DC potential method, solving the influence of temperature change, welding spot oxidation and thermoelectric effect on the measurement results under high temperature test condition. The crack length measured by the DC potential method is calibrated by the overload line method. The results show that the error curves show a linear trend when calibrating the compact tensile specimens and the center tensile specimens, the measurement error of the compact tensile specimens is within 18% and the measurement error of the center tensile specimens is within 4.3%. The results measured by the potential method are compared with those measured by the overload line method. The results measured by the potential method have linear errors. The accurate crack length can be obtained by linear correction, and thus the propagation rate data can be calculated.