Abstract: In order to research new sensitive materials, a mathematic piezoresistivity model of the conductive composites filled by carbon black particles is presented on the basis of general effective medium (GEM) theory . The model quantitatively describes the effects of the carbon particles size, volume fraction and polymer's elastic modulus. Some experimental samples were fabricated by dispersing three carbon black particles with different sizes in silicone rubber and high density polyethylene matrix respectively. It is proved in experiments that the mathematic piezoresistivity model is consonant with the experimental results very well under three boundary conditions: The carbon particles are dispersed homogeneously, the volume fraction of carbon black is near by the critical threshold, and the applied pressure is not more than 2 MPa.