The piezoresistive behavior of carbon black(CB) filled poly(methyl vinyl siloxane) (PMVS) vulcanites under uniaxial compression was studied. At filler weight fractions φ slightly above the percolation threshold φc, resistance first increased with pressure and then turned to decrease at a critical compressive stress, exhibiting a positive pressure coefficient of resistance (PPCR) and a negative pressure coefficient of resistance (NPCR) effects, respectively.The NPCR effect became much more pronounced at φ＞＞φc,while compressive cycles facilitated the occurrence of the weak PPCR effect during compression. The PPCR-NPCR transition was a process related to true stress. It is believed that the changes of microstructure in the percolating network, i.e. the breakdown and the reformation of infinite conducting clusters, under pressure would be responsible for the uniaxial piezoresistance and the plastic deformation of the filled vulcanites.