This paper describes the synthesis of Ni-Zn nanoferrites by combustion reaction using microwave energy as a heating source, and evaluates the performance of these materials as absorbers of electromagnetic energy at frequencies between 4 - 12 GHz. The influence of the synthesis conditions on the structure, morphology and absorption characteristics was investigated. The powders were characterized by DRX, BET, AGM and reflectivity measurements in the frequency bands of 8 to 12 GHz. The XRD results show the formation of Ni-Zn ferrite phase and Fe2O3 and Ni as secondary phases. The crystallite sizes ranged from 32 to 42 nm. The parameters of exposure time and power of the microwave oven changed the final characteristics of the resulting powders. The morphology of all the powders consisted of soft nanoparticle agglomerates. The best saturation magnetization and attenuation results were 70 emu/g and -4.1 dB in the frequency of 10 GHZ.