The kinetics of isothermal crystallization for high-density polyethylene (HDPE) containing different volume fraction (phi) of carbon black (CB) have been evaluated by using differential scanning calorimetry (DSC) at 123, 124, and 125 degrees C where the rate of crystallization is moderate. Simultaneous measurement of normal force (F-N) and electrical resistance (R) has been performed to probe the process of isothermal crystallization at strain zero. Results reveal that, at the early stage of crystallization, F-N is almost independent of time (t) while relative resistance (R/R-0) changes slightly with increasing time t. However, a significant increment in F-N and a remarkable change in R/R-0 can be observed at the same critical time (t(c)), and the value of t (c) is dependent on the crystallization temperature and CB content, which is available for describing the isothermal crystallization as a characteristic parameter. It is found that t (c) is greater than induction time of crystallization (t(0)) due to the less sensitivity of mechanical and electrical responses than enthalpy to the structural changes in the composites. It is suggested that mechanical and electrical simultaneous measurement endows us a novel approach to probing the formation of percolation network involving in crystallization of polymer matrix.