To study loss mechanisms of magnetic loss radar absorbing coatings （RACs）， two kinds of carbonyl iron powders and a kind of ferrite were used. The permeability and permittivity were tested by coaxial method， the attenuation constant， phase constant， wavelength， and attenuation loss of microwave with frequencies at 1~18 GHz in RACs were investigated. And the input impedance of RACs on microwave were calculated. The results show that the attenuation loss of microwave at 18 GHz by type S absorbing coating with thickness of 2 mm is -24.9 dB， which shows the electromagnetic energy is reduced to only 0.3% of the original energy. These frequencies of maximum of real input impedance Re（Z_in） of RACs stay the same with frequencies of maximum of reflection loss of radar absorbing materials separately. The impedance matching of real input impedance Re（Z_in） of RACs with intrinsic impedance of free space is not the only factor of reflection loss of radar absorbing coatings.