Mossbauer measurements and high-resolution electron microscopy have been carried out on rapidly quenched 10CaO . 30Bi2O3 . 6OFe2O3 and Bi3Fe5O12 which exhibit ferromagnetic character at room temperature. The intensity of the peaks due to internal fields in Mossbauer spectra are very weak at room temperature and increases with a decrease of temperature, indicating that the specimens are superparamagnetic. The electron micrograph clearly shows that the specimens contain microcrystals of 5 to 10 nm. The ferromagnetic character of the present oxides is brought about by the ferrimagnetic microcrystals precipitated.