Abstract: The electrical conductivity and its specific resistance varying with compressive stress of cement based composite materials (CBCM),reinforced by carbon fiber, black lead and steel fiber, were studied. The changing mechanism of specific resistance of CBCM,mixed with electric admixtures, was also researched. The results show that the precondition of CBCM possessing the piezoresistive effect is its electric net reaching the vadose zone. Moreover, due to the carbon fiber possessing a character of high slenderness ratio and low density, only a small content of it is enough to make CBCM enter the vadose zone and possess the piezoresistive effect.