Eddy current testing was applied for the quantitative evaluation of the matrix structure and hardness of ductile cast iron. Two kinds of specimen groups, (1) 19 specimens of the 5-step bar type and (2) 57 specimens cut from actual casting products, were prepared. Three types of eddy current equipments, (1) Commercial eddy current tester, (2) Special tester designed for cast iron characterization, (3) Simple tester using an iron core coil were used. The correlation coefficient (R) between Brinell hardness and eddy current signal of 50 kHz was as high as 0.92 for all the specimens, and 0.96 for the selected specimens having similar chemical composition and graphite shape. The specially designed tester, which is easy to handle and inexpensive, showed almost the same accuracy as the commercial eddy current tester. Distortion of the eddy current signal caused by strong magnetic field with an iron core coil was larger in ferritic cast iron than in pearlitic cast iron. This discrepancy was detected through a high pass filter and successfully applied to evaluate the hardness of cast iron.