CoFe 2 O 4 nanoparticles with a cubic spinel structure are prepared by a high-temperature thermal decomposition method. The average particle sizes are 4.6 and 5.7nm for CoFe 2 O 4 made with two kinds of solvents by TEM. Mössbauer spectra of 4.6nm particles displayed a superparamagnetic behavior as demonstrated by a single line with zero hyperfine fields, but that of 5.7nm particles did not at room temperature. It is considered that anisotropy energy was still more superior to thermal energy because of particle size of 5.7nm CoFe 2 O 4 . Furthermore, Mössbauer spectra exhibited the typical spectrum shapes of the CoFe 2 O 4 at 4.2K. The spectrum at 4.2K was fitted using two magnetic components of hyperfine fields Hhf=540.4,512.6kOe and isomer shifts δ=0.40,0.30mm/s for 4.6nm and Hhf=542.7,512.8kOe and δ=0.41,0.29mm/s for 5.7nm corresponding to Fe 3+ ions at site A and site B, respectively.