Carbon black (CB)-loaded high-density polyethylene composites were prepared using conventional blending. The resistance and temperature (R-T) relations under constant heating rates and the resistance and time (R-t) relations at different isothermal temperatures have been studied. The results of the R-T and differential scanning calorimetry (DSC) curve demonstrated a correlation between the positive temperature coefficient/negative temperature coefficient transition and the melting course. At isothermal temperatures below T PTC/NTC , the resistance displayed a sharp increase and thereafter a mild decrease with time. The time to reach the highest resistance became shorter with rise in the isothermal temperature. The ratio between highest resistance and initial resistance was maximum at T peak of the DSC curve. When the isothermal temperature was higher than T PTC/NTC , the resistance attenuated with time. The attenuation fits to a first order exponential decay function. The calculated time constant decreased with rise in isothermal temperature. The attenuation discrepancy under different isothermal temperatures reduced as the heating rate before the isothermal courses was higher. A model based on polymer chain diffusion and CB movements at high temperature is proposed. The model can explain the results obtained in R-T and R-t measurements.