The damage characteristics of ferrite ductile cast-iron under uniaxial stress has been studied through measuring electric resistance. The results showed that the damage in ductile cast-iron happens when the applied stress is more than a certain extent, and the damage value increases with stress. The evolutional law of damage value as a function of stress was obtained. The damage threshold of ductile cast-iron exists and increases with nodularity, but this threshold is always below the yield strength, which has related significant reference to the design of machinery structure and the choice of materials. The critical damage value is not related to the nodularity, which is about 0.06~0.068 under the present test condition.