The mechanism of the hydration process of cement paste, mortar and ordinary concrete has been studied by AC impedance technique. Different stages of the hydration process of cementbased materials were classified by the characteristics of the impedance spectra. The hydration of cementbased materials were also classified as primary, secondary and tertiary according to the binding form of the material with the water. The negative capacitance occurring in the initial and early stages of hydration has been interpreted from the viewpoint of structure.