The ${\mathrm{Fe}}_3{\mathrm{O}}_4$ ferrofluids samples were synthesized by a co‐precipitation method. The investigation of ferrofluids microstructure is known to be one of the most important problems because the presence of aggregates and their internal structure influence greatly the properties of ferrofluids. The size and the size dispersion of particle in ferrofluids were determined assuming a log normal distribution of particle radius. The scattering pattern of the small angle neutron scattering measurement were fitted by the theoretical scattering function of two limitation models are log normal sphere distribution and fractal aggregate. Two types of particle are detected, which are presumably primary particle of 30 Å in radius and secondary fractal aggregate of 200 Å with polydispersity of 0.47 up to 0.63.