A study was carried out on the variation in the electrical resistance of carbon fiber-reinforced unsaturated polyester/polyurethane (UP/PU) IPN composites during static tension, cyclical loading, and unloading in tension. The resistance was found to vary nonlinearly with strain and increase abruptly when the composite approached the fracture. Under a certain constant tension, the change in electrical resistance showed time dependence, and it is suggested that the creep of the IPN network imposed an impact on the variation of resistance. The resistance probing revealed that carbon fiber-reinforced plastic can function as a sensor that is capable of self-diagnosing the fatal fracture in composites.