The van der Pauw technique is one of the popular methods for determining the conductivity of flat metal samples. Traceability of the national standard for the conductivity has been achieved by direct current measurement techniques in national measurement institutes of many countries. But recently, alternative current measurement techniques for determining the conductivity of flat metal samples is also interested. In this study, we measured the conductivity of non-ferrous and ferrous flat metals at alternative current using van der Pauw technique. As measurement results, the conductivities of the samples were decreased according to increasing the test frequency even though the decreasing ratio was different.