The dc conductive properties of oxidized polyacrylonitrile-based electrorheological (ER) fluid under zero and oscillatory mechanical fields are examined. In a zero mechanical field, the resulting de current through the ER fluid decreases approximately exponentially with time to a limited value once the applied electric field strength exceeds a critical value, whereas in an oscillatory mechanical field, the de current, obtained below the critical electric field, changes with the applied mechanical frequency and strain amplitude. The conductance results measured under the off-state mechanical field are in accord with the quasi-one-dimensional variable range hopping model of charge carriers localized along a one-dimensional chain. A modified fibrillated chain structure model is presented to interpret the conductive behavior. The conductivity of this ER suspension is thought to depend on the number and nature of the continuous and the isolated conductive chains accumulated by the dispersed particles, A percolation theory is introduced to analyze our results.