Electrical impedance measurements wereperformed on fine-grained concrete with low,volume con-tent of conductive steel and carbon fibres,either as a monoor as a hybrid system.The influences were investhated ofthe applied frequencies,of fibre combinations and of theage of the composites on the impedance.the results showthat when the applied frequency is increased from 0. 98Hzto 1000Hz,the impedance of the fibre composites significantly decreases.At the same time,impedance differencebetween plain concrete and fibre-reinforced concrete is en-hanced.Dramatic drop of impedance takes phace over thelow,frequency range,i.e.0. 98～31Hz.Impedance alsodepends on the volume content of hybrid fibres.The cemen-titous composite with the highest content of hybrid fibres(1. 0% steel fibres and 0. 2% carbon firbres by volume)has the lowest impedance at all measuring frequencics.0.6% fribres by volume seem to be a turming point,fromwhich the variation of impedance slows down with the fibre content increases.On the other han Electrical impedance measurements were performed on fine-grained concrete with low, volume con-tent of conductive steel and carbon fibers, either as a monoor as a hybrid system. These influences were investhated of the applied frequencies, of fiber combinations and of the of the composites on the impedance. the results showthat when the applied frequency is increased from 0. 98Hz to 1000Hz, the impedance of the composites significantly reduces .At the same time, impedance differencebetween plain concrete and fiber-reinforced concrete is en-hanced. Dramatic drop of impedance takes phace over the low frequency range, ie0. 98 ~ 31 Hz. Impedance alsodepends on the volume content of hybrid fibers. cemen-titous composite with the highest content of hybrid fibers (1.0% steel fibers and 0.2% carbon firbres by volume) has the lowest impedance at all of measuring frequencics.0.6% fribres by volume seem to be a turming point, fromwhich the variation of impedance slows down with the fiber content increa ses.On the other han