Abstract: The preparation of CoFe2O4 ferrofluids by Massart's method is discussed in this paper. The structure and the shape of CoFe2O4 nanoparticles are characterized by X-ray diffraction and transmission electron microscopy, respectively, and the results show that the particles' size follows a logarithmic normal distribution, with the average diameter of 12.76nm and the crystal particle's average diameter is 11.15nm. The magnetization curve of CoFe2O4 particles is determined at room temperature by VSM, with the saturation magnetization of CoFe2O4 particles of 217.50kA/m. The magnetization curves of ferrofluids with volume fractions of 1%, 1.2% and 1.5% are determined at room temperature by VSM. It is found that the saturation magnetization of ferrofluids is significantly less than particle's saturation magnetization fVMps, because of bulk aggregations, chain-like aggregations and ring-like aggregations in ferrofluids at the same time. The bulk aggregations and the ring-like aggregations reduce the saturation magnetization of ferrorluids. The saturation magnetization of ferrofluids after frozen is greater than that before frozen, as the ring-like aggregations are broken by H2O molecule's interaction during the frozen process, which increases the parameter γ and the saturation magnetization of ferrofluids after frozen.