The changes in electrical resistance of carbon fibers during a tensile elongation were investigated to understand the electromechanical mechanism in carbon fibers. The fractional electrical resistance of carbon fibers initially increased slightly with increasing elongation, however, increased abruptly beyond a certain strain where the rupture of fibers began to increase. Contribution to this change in electrical resistance was analyzed in terms of dimensional change of fibers, number of ruptured fibers, and degree of fiber contacts. The effect of the number of ruptured fibers was the most dominant, whereas the effect of the dimensional change of carbon fibers due to elongation was relatively small. The degree of contacts between fibers affected the change in electrical resistance dominantly at the large elongation. The residual electrical resistance appeared upon removal of the applied strain and increased with increasing elongation, regardless of the static and dynamic loading. Consequently, the smart characteristics of carbon fibers due to the existence of the residual electrical resistance are primarily ascribed to the number of ruptured fibers and contacts between fibers.