The effect of a plasticizer on the change of the electrical resistance under deformation at stretch is studied in polyisoprene (PI) composites containing dispersed conducting nano-size carbon particles at concentrations in the vicinity of percolation threshold. A change of the electrical resistance by more than four orders of magnitude has been earlier observed at 40% stretch of the composite containing 10 mass parts of carbon. Addition of plasticizer to the initial materials makes the composite more sensitive and broadens the percolation threshold. The observed improvements of the tenso-resistive response are explained by diminished cohesion and higher mobility of the carbon nano-particles (CNP) in the elastomer matrix in presence of the plasticizer.