Abstract: The condition of fibers in carbon fiber sheet with epoxy resin mat rix used in civil inf rast ructure is different from that in indust rialized carbon fiber reinforced polymer/ plastic (CFRP) , i1 e. some fibers located inner ofthe fiber bundle are indeed without epoxy resin mat rix. The self-diagnostic function of carbon fiber sheet without epoxy resin mat rix was studied under a tensile loading condition. The experimental results indicate that the resistanceof carbon fiber sheet without epoxy resin mat rix linearly changes with the applied st rain , so the carbon fiber sheetcan act as a sensor to measure strain. A parallel variant2st ructure circuit , in which each carbon fiber bundle playsone path and a resistor , is employed to model the elect ric behavior of carbon fiber sheet and the resistance changescaused by the elongation and breakage of carbon fibers one by one. It is found that the periphery carbon fibers firstlybreak and cause an abrupt increase of resistance of carbon fiber sheet through changing the st ructure of the circuit ,and then the broken carbon fibers do not cont ribute to the resistance of carbon fibers anymore. The carbon fiberslocated inner of carbon fiber bundles also break later and still contact with the unbroken fibers at one point or severalpoint s , thus , these kinds of broken fibers carry on paths of the parallel circuit and still cont ribute to the resistance ofthe carbon fiber sheet . The ratio of un-contact broken fibers to all broken fibers was identified by fitting the testdata. The calculated results by using the variant2st ructure parallel circuit agree well with the test results.