Heat-treatments were carried out on carbon steels ranging from 0.03 to 0.78 wt%C, in order to produce various ferrite-pearlite microstructures. The specimens were characterized by metallographic examinations and microhardness measurements. A clear dependence was found between microstructure characteristics and eddy current outputs measured by means of an electromagnetic sensor: resistance was observed to increase, while inductive reactance decreased in the order of pearlite and ferrite microstructures, and with decreasing interlamellar spacing of pearlite. These components are related to the electrical resistivity and magnetic permeability of the steels. The potentiality of this technique was highlighted for monitoring phase proportions, quantitatively assessing pearlite interlamellar spacing, giving also information about mechanical properties, such as hardness. It reveals the great potential of eddy current testing as a reliable non-destructive tool for metallurgical and mechanical characterization of carbon steels.