BaFe12-xNbxO19 (BFNO, x=0-0.6) powders with Nb5+ substituting for Fe3+ were prepared by sol-gel method. The formation process and electromagnetic (EM) wave absorption properties of the BFNO are investigated in detail. With Nb5+ content increasing from x=0 to x=0.6, the formation temperature of barium ferrite phase without heat time increases from similar to 700 degrees C to similar to 900 degrees C, while the appearance temperature of typical plate grains decreases from similar to 1300 degrees C to similar to 1100 degrees C, and the crystallization ability decreases at 600 degrees C-900 degrees C, while the grain size increases gradually at 1100 degrees C-1300 degrees C. Increasing sintering temperature and time promote the formation of barium ferrite phase and grain growth in all the samples. The epsilon and epsilon of the sample with x=0.6 sintered at 1300 degrees C for 3hours reach highest of similar to 7.9 and similar to 0.95 over 26.5-40GHz. Multiresonance peaks in permeability decrease from 40+ GHz to similar to 30GHz with x rising from 0 to 0.6. Ultimately, small RLmin of similar to-42dB, thin d(m) of similar to 0.76mm, and broad bandwidth of >12GHz can be exhibited simultaneously around millimeter wave atmospheric window of 35GHz.