Nanocrystalline bismuth orthoferrite was synthesized by heat treatment of hydroxides co-precipitated in a free impinging-jets microreactor (FIJMR). The formation of BiFeO₃ nanoparticles without other phase impurities, with a narrow crystal size distribution and an average crystal size of about 20 nm, is shown to be possible after heat treatment at 420–440 °C of the hydroxides co-precipitated in a FIJMR. The BiFeO₃ nanoparticles obtained under these conditions are single-crystalline. This effect is attributed to the crucial intensification of mixing process compared to the usual types of reactors: the energy dissipation rate for FIJMR estimated as being in the range of 10⁷–10⁸ W/kg, which is several orders of magnitude higher than in almost all types of reactors, and is comparable with the level achieved when using ultrasonic generators. The sizes of the nanocrystals are shown to be consistent with the sizes of the nanoparticles that can be formed in Kolmogorov microvortices that are formed when jets of reagent solutions collide.