The thermal history of carbon-fiber nylon-6 matrix composite was evaluated by measuring the electrical resistance in the fiber direction during heating, as the resistance was affected by the degree of crystallinity and the thermal oxidative degradation. The resistance results were consistent with different scanning calorimetry (DSC) and thermomechanical analysis (TMA) results. The resistance measurement gave more information on the melting of the polymer-matrix than TMA. In particular, it distinguished more clearly between the melting of the reorganized crystalline phase and that of the original crystalline phase. The resistance measurement is more sensitive to the glass transition of the polymer-matrix than DSC.