In order to understand microstructure characteristics and properties of reactive powder cementitious materials with low w/b, the A.C impedance test on reactive powder cementitious materials is carried out. The correlation between the impedance parameters, microstructure and properties of reactive powder cementitious materials is also analyzed. Results indicate that the impedance-frequency curve both for reactive powder mortar and ordinary cement mortar consists of vertical part in range of low frequency section and horizontal part within high frequency section. But the angle between vertical part and horizontal part of curve for reactive powder mortar is larger compared with ordinary specimen. There is a good linear relationship between the real part of impedance and angular frequency ω-1/2 for mortar when frequency is from 10Hz to 100Hz. And the slope φ of line increases with the additions of reactive powder and the lowering w/b. The mechanical strength and microstructure of reactive powder cementitious materials with low w/b can be expressed well by the A.C impedance characteristic parameters.