Hard magnetic strontium hexaferrite SrFe 12 O 19 nanoparticles were synthesized by the sol–gel hydrothermal method. The factors affecting the synthesized process, such as the mole proportion of the reactants, pH, temperature, the hydrothermal conditions and the calcination process, have been investigated. The crystal structures of these materials were refined by Rietveld method. The obtained materials have single crystal phase, equal nano-size, plate shape and high anisotropy. The high magnetic coercivity of 6.3 kOe with the magnetization at 11.1 kOe of 66 emu  g −1 at room temperature was observed for the strontium hexaferrite nanoparticles. For other nanoparticles (SrLn x Fe 12-x O 19 and SrFe 12 O 19/CoFe 2 O 4) synthesized on the basis of SrFe 12 O 19 the complex completion of the crystal structure distortion and the interaction between magnetic phases were observed.