Abstract: When conductive particles are dispersed beyond the percolation threshold, electric conductive paths are formed between the electrode by chains of particles contacting with each other between the electrodes. Elongation of this composite results in an increase in the gap distances between conductive particles. This results in the increase in the electric resistance of the composites. It is found that strain sensors can be made by the use of this nature. Strains of iron frames or iron-concrete are known by the change of electric resistance of the sensors which are set on a surface of the place to be monitored. Main fields of the application of the present sensors are safety monitoring systems for buildings, bridges, tunnels, dams, etc. The sensors are also applicable for tanks of chemicals, aircraft, ships and mega-floats.