The potential drop method is applied to the measurement of fatigue cracks in a gear tooth. First, the relationship between the direct current potential drop and the crack length is calculated by the boundary element method (BEM), and the influence of the positional errors of terminals, probes and crack on the potential drop is discussed. From this result, an expression is obtained for estimating the crack length a from the normalized potential drop V/V0. The alternating current potential drop V is measured during fatigue crack growth in a gear tooth. The potential drop V is approximately proportional to the square root of the current frequency f. The normalized potential drop V/V0, which is defined using the potential drop V0 for an uncracked tooth after loading, is fairly close to the result calculated by BEM. The value V/V0 slightly depends on the current frequency f.