CoFe2O4 nanoparticles that were synthesized by coprecipitation method were heat-treated at different temperature, and analyzed by XRD and TEM. The particle size and magnetic properties of CoFe2 O4 nanoparticles also was tested. The results showed that the average sizes became large as the heat-treated temperature increased, the saturation magnetization and remanent magnetization increased with the heat- treated temperature too.