The hydration process of cement pastes was investigated by using non contact impedance measurement(NCIM). The reliability of impedance data from NCIM was checked by classical Kramers Kronig transformation. Hydration behaviors of pure cement paste in different stages(dissolution, competition and acceleration stages) were interpreted by NCIM. It is found that the impedance response of cement pastes is approximately resistive in the dissolution and competition stage. Experimental and calculated imaginary parts of impedance of cement pastes in the acceleration stage increases as frequency increases. Furthermore, the early age compressive strength of cement pastes and impedance modulus have a good linear relationship.