A study of the effect of both plasticizer and streching strain on the percolation transition in polyisoprene-carbon nanocomposites (PCNC) is reported. The orthopositronium (oPs) accessible free volume sites in the unstreched and streched PCNC without plasticizer as well as with plasticizer have been measured by positron annihilation lifetime spectroscopy (PALS). The lifetime of oPs, 3, is related to the size of the free volume, and the intensity, I3, to the number concentration of free volume sites. It was found that streching decreases the number of free volume cavities independent of the both mass parts carbon nano-particles (CNP) and plasticizer. The free volume cavity size has maximum value in the region of percolation transition. Percolation threshold was determined by electric resistance measurements (ERM). Both PALS and ERM show that the percolation threshold shifts to greater values of mass parts of CNP under streching strain. An opposite shift of percolation threshold was found under adding plasticizer. It is interesting that adding of CNP increases mean free volume cavity size at mass parts < percolation threshold. Decrease in cavity number (I3) was observed at mass parts of CNP > percolation threshold. It means that filling of cavities occurs above the percolation threshold. The fatigue of tenso-resistive effect for PCNC samples, containing 10 m.p. of CNP with plasticizer added and without plasticizer has been examined.