Ultrafine cobalt ferrite particles have been sythesized using a microemulsion method. All peaks of X-ray diffraction patterns are fairly broad but correspond to a cubic spinel structure with the lattice constant 8.39 Å. The coercivity measured at 5 K is 15.1 kOe. The maximal magnetizations measured at 5 and 300 K are 13.2 and 10.7 emu/g, respectively. The particles behave ferrimagnetically at 5 K but superparamagnetically at 300 K. Superparamagnetic behavior of the particles at room temperature was confirmed by the coincidence of the M vs. H/T at different temperatures and the Mossbauer spectrum.