By means of SANS there were investigated Fe(3)O(4) D-benzene and heavy water based ferrofluids with different particle concentration. It was obtained that the thickness of surfactant shell is decreasing with the increase of the particle concentration, while the degree of the solvent penetration in the surfactant shell is constant. These results are in agreement with those obtained by viscosity measurements. Using the Guinier approximation for the heavy water based ferrofluid samples it was further obtained the radius of gyration for each concentration. So, it was found that beginning with the 1:3 dilution of the heavy water ferrofluid sample the microstructure of ferrofluid sample changes.