In this work, nanoscale powders of Co- and Ni-substituted barium hexaferrites were obtained by hydrothermal synthesis (BaFe_12-xCo_xO₁₉ and BaFe_12-xNi_xO₁₉ with x=0.1, 0.3, 0.5). The obtained samples were analyzed by several methods, including XRD, EDX, VSM, FTIR and TEM. It is shown that despite the close chemical nature of Co²⁺ and Ni²⁺, the effect of substitution of Fe³⁺ by these elements is completely different. Thus, nickel has virtually no effect on the shape and size of the resulting BaFe_12-xNi_xO₁₉ particles (plate-shaped crystallites with a diameter of about 200 nm and a thickness of 60 nm). With an increase in the nickel concentration, the magnetic parameters of the resulting ferrites decrease almost linearly. Cobalt, on the contrary, in a certain concentration leads to a significant change in the morphology of the particles (thinning of the crystallites to 30 nm or less), resulting in a sharp decrease in the coercive force of the resulting powders. This effect is due to the fact that cobalt promotes the formation of the BaFe_12-xCo_xO₁₉ phase directly during the hydrothermal treatment of the precursors, whereas additional high-temperature treatment is required to form BaFe_12-xNi_xO₁₉ and BaFe₁₂O₁₉. Keywords: barium hexaferrite, hydrothermal synthesis, substituted hexaferrite, Mossbauer spectroscopy, magnetic measurements.