Fe-Ni/C composite nanofibers with an average diameter of about 215 nm were fabricated via an in-situ decomposition and reduction of electrospun polyacrylonitrile/iron acetylacetonate/nickel acetylacetonate precursor fibers during the thermal stabilization and carborization processes. The Fe-Ni alloy nanoparticles in-situ generated during carbonization are well distributed on the surface and inside the nanofibers and encapsulated mostly by the ordered graphitic layers, which can avoid the direct exposure of Fe-Ni alloy phase to air and preserve the structural and interfacial stabilization of Fe-Ni alloy nanoparticles. By dispersing the Fe-Ni/C composite nanofibers homogeneously into a silicone rubber matrix, their electromagnetic parameters were measured in the frequency range of 2-18 GHz and the influences of carbonization temperatures on the electromagnetic characteristics and microwave absorption properties were investigated. It is found that the absorbing coatings containing only 5% (mass fraction) Fe-Ni/C composite nanofibers as fillers with thickness of 1.2-2.0 mm exhibit excellent electromagnetic wave absorbing performances, and the reflection loss(RL) exceeding 20 dB can be obtained over the range of 7.4-18 GHz. The microwave absorption properties for the as-prepared nanocomposites are gradually enhanced with increasing the carbonization temperature from 800 degrees C to 1200 degrees C due to the better impedance match, accompanying with a decrease of the minimum RL values from -22.6 dB to -63.0 dB. Overall, the results reveal that the as-prepared Fe-Ni/C composite nanofibers can be a promising candidate for microwave absorbing materials with strong-absorption, light-weight and thin-thickness in X and Ku bands.