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Attenuation of conduction noise through microstrip line attached with high lossy iron flakes-rubber composites has been investigated in gigahertz frequencies. Microstrip line was designed with characteristic impedance of 50 Ω and a length corresponding to the center frequency of 3 GHz. Iron flakes were fabricated by mechanical forging of spherical iron powders using an attrition mill. The fabricated microstrip line shows an ideal propagation characteristics of S11=- 60 dB and S21= 0 dB. Attaching a noise-absorbing sheet on the microstrip line, S11 increases to about - 10 dB and S21 decreases to -20-- 60 dB, depending on the size of the composite sheet. The calculated power loss is as high as 90% in the frequency range 2-8 GHz. Noise-absorbing characteristics was explained as a function of frequency, sheet size covering the transmission line, and material loss of the composite sheets.