Abstract: Principle, test method and instruments of direct current potential drop method (DCPD) to test the fatigue cracks length were introduced and influencing factors to the test were also analyzed. DCPD adapted to automatics high temperature fatigue crack length measurements was investigated using Johnson’s formula. The DCPD method was successfully applied to high temperature fatigue crack growth test for superalloy. The da/dN-ΔK test data obtained by DCPD method were exactly the same as them obtained by long distance of microscopy method.