Nanocrystalline strontium hexaferrite (SrFe12O19) and barium hexaferrite (BaFe12O19) powders were synthesized by co-precipitation method. The ‘as synthesized’ powders were heat treated (HT) at different temperatures ranging from 800 to 1200°C at a heating rate of 30°C /min in nitrogen atmosphere. Decomposition behaviour and the phases associated therein are investigated by thermal analysis (DTA/DTG/TG) and X-ray diffraction (XRD). Formations of ultrafine particles have been confirmed through field emission scanning electron microscop (FESEM). The superparamagnetic behavior of both, barium and strontium hexaferrite is confirmed by vibrating sample magnetometer (VSM). The increase in saturation magnetization from 1.94 to 31.05 emu/gm in case of barium hexaferrite and from 2.44 to 43.38 emu/gm for strontium hexaferrite is observed with HT temperatures. The changes in coercivity and remanence with HT temperatures for both the ferrites are analysed.