The nanocomposite BaFe12O19 (BFO)/Ni0.5Zn0.5Fe2O4 (NZFO)-aligned microfibers with diameters 0.8–2 μm and high aspect ratios have been prepared by the gel precursor transformation process, where BFO is the hard magnetic phase and NZFO the soft magnetic phase. The nanocomposite binary ferrites are formed after the gel precursor calcined at 1000°C for 3 h. These nanocomposite aligned microfibers exhibit a clear shape anisotropy. Their magnetic difference observed at 297 and 77 K largely arises from the competition of exchange-coupling interaction and thermal fluctuation on dipolar interaction. Consequently, their microwave absorption performance is largely influenced by the mass ratio of BFO/NZFO and specimen thickness. When the mass ratio (BFO:NZFO) is 7:3 and the specimen thickness is 3 mm, the minimum reflection loss (RL) reaches the optimized RL value of −35.5 dB at 12.4 GHz, with a wide absorption bandwidth (the RL value over −20 dB) from 9.1 to 15.7 GHz, which covers 80% of X-band (8.2–12.4 GHz) and 59% of Ku-band (12.4–18.0 GHz). This improvement in microwave absorption can be attributed to the exchange-coupling interaction, shape anisotropy, interfacial polarization, and small size effect in nanocomposite microfibers.