The strain sensor can be used to check the safety of iron frames or iron-concrete in buildings based on reading changes in the electrical resistance of sensors located on the surface of the area being monitored. When the sensor is extended, the gaps between the carbon black particles in the polymer matrix become significantly large, thereby linearly increasing resistance of the sensor linearly increases. HDPE, LDPE and EVA were selected as the polymer matrices in current study. The effect of the particle size, surface area, and carbon black content on the electric resistance was studied. Dumbbell shaped samples were cut from a composite sheet, strained, then the electrical resistance and elongation measured simultaneously. It was found that the relation between the electrical resistance and the elongation of the sensor depended on the particle size, surface area, carbon black content, and type of polymer. A polymer sensor filled with carbon black with a large particle size and low surface area exhibited a sensitive electrical resistance to strain.