The influences of carbon black (CB) structure on the percolation threshold, mechanical properties, and strain-resistivity response of polymer composites are studied. Low-density polyethylene (LDPE) and polypropylene (PP) samples were blended with five different types of CB differing in structure. Relatively low strains were studied; the maximum strain was 10%. It was found that the CB concentration for maximum strain-sensitivity of the electrical conductivity is higher for low structure carbon blacks but is essentially independent of the CB structure for medium- to high-structure carbon blacks. However, the composite containing the largest particle size carbon black clearly showed the highest strain-sensitivity to electrical conduction. The mechanical properties and sensitivity of electrical resistivity to tensile strain of the filled composites examined in the study are also presented and discussed. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers