The interphase thickness in polymer carbon nanotube (CNT) nanocomposites (PCNT) is expressed by CNT characteristics and interfacial conductivity between CNT and polymer matrix. Also, the effective length and concentration of CNT are defined supposing interfacial conductivity to suggest the percolation threshold and the fraction of networked CNT. Furthermore, a simple model is developed to predict the conductivity of PCNT assuming tunneling effect, interphase region and interfacial conductivity. The influences of all parameters on the interphase thickness, percolation threshold, the fraction of networked CNT and the conductivity of PCNT are studied. In addition, some experimental results are provided to compare the predictions of developed model to the experimental measurements. The high levels of CNT radius, CNT length, CNT conductivity, interfacial conductivity and conductivity transferring can produce a thick interphase, but the waviness cannot change it. Moreover, the developed model can properly calculate the conductivity of studied samples. A desirable conductivity is obtained by thin and long CNT, medium CNT conductivity, poor waviness, high interfacial conductivity and good conductivity transferring, because they provide high effective CNT concentration and low percolation threshold in nanocomposites.