The impedance spectra of hydrating Portland cement paste showed a small part of a large depressed arc and a single depressed arc at low- and high-frequency regions, respectively. A new equivalent circuit was proposed for hydrating cement paste, which consists of a serial combination of a resistor and a constant phase element (CPE) with a resistor in parallel for the bulk effects, and a parallel combination of a resistor and a CPE for the electrode effects. The microstructural evolution in hydrating cement paste ws associated with the changes in the resistances, capacitance and the critical relaxation time.