To investigate the stretching effect of polymer-based composites, the films of polyvinylidene fluoride (PVDF) and nickel (Ni) composites were stretched at 120 degrees C. It was found there was an abrupt increase in dielectric constants and conductivities with stretching ratio (SR). In comparison to percolation effect caused by involvement of conductive particles, it was named as stretching-induced percolation here. When SR was near the threshold, dielectric constants showed 17 times improvement and conductivities displayed four to five orders of magnitude enhancement at 100 Hz in low Ni fraction composites. And the reasons of stretching-induced percolation were analyzed here. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 2368-2373, 2011