This study investigated the feasibility of ultrafast crystallization of M-type barium ferrite when the coprecipitation precursors in stoichiometric proportions as BaFe12O19, Fe(OH)3 and BaCO3 nanoparticles, had been heated by spark plasma sintering (SPS) process. The results show that SPS method may realize the ultrafast crystallization of M-type barium ferrite, absolutely prevent the crystallization of intermediate phase α-Fe2O3, and significantly decrease the crystallization temperature of M-type barium ferrite. The sintered samples obtained at 800℃ by sintering the precursors for 10 minutes are a kind of multiphase ferrites composed of major phase M-type barium ferrite and trace amount of BaFe0.24Fe0.76O2.88. It is discovered that M-type barium ferrites in the holes of the sintered samples are in nanobelt microstructure about 100-300 nm in width and several micrometers in length. These M-type barium ferrite nanobelts are non-stoichiometric and may be expressed as BaFe12+Xo19+1.5x (-4.77≤x≤6.50). Their composistions suggest completely random Fe-rich or Ba-rich domains.