Nanosize particles of SrFe12O19 have been synthesized by a citrate precursor technique with various Fe/Sr ratios of 12.0 to 9.0. X-ray diffraction (XRD) studies showed the formation of ultrafine, monophasic SrFe12O19 particles with comparable unit cell parameters. Magnetic clusters were observed by scanning electron microscopy and Branauer–Emmett–Teller surface area measurements. Mossbauer results reveal the ferrimagnetic and superparamagnetic nature and the change in hyperfine parameters with decline in particle size and Fe/Sr ratios. The magnetic properties of small SrFe12O19 particles (Fe/Sr=12.0) showed a considerable enhancement in TC from a bulk value of 750–883 K measured for 20–5.7 nm XRD particle size. The shift in Curie temperature is well defined by the finite size scaling formula [TC(D)-TC(∞)]TC(∞)-1=(D/D0)-1/ν with ν=0.75–0.02 and D0=1.52 nm. The change in Hic, σs, and σr values were observed with the decline in particle size and the Fe/Sr ratios where Hic of 6500 Oe is achieved for isotropic SrFe12O19 particles with a Fe/Sr ratio of 10.8. Such changes are explained by the additional Sr leading to the formation- of a nonmagnetic layer around the particles which inhibits their growth, limiting them to single-domain size. © 1998 American Institute of Physics.