Size and correlation analyses of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanoparticles have been carried out by the measurement of small angle neutron scattering. Particle size of magnetite is analyzed by employing Guinier region approximation in order to obtain gyration radius, which will be used to further determine the average radius of magnetite particles resulting in the value between 17.8 nm and 53.6 nm. Further approximation using a polycore‐shell ratio model shows the average particle size of 25 nm with polydispersity of 0.4. The effect of magnetic fluids concentration on the thickness of surfactant layer on the particles surface has not shown a significant change with the value between 6 Ǻ and 8 Ǻ relating to the molar concentration between 0.5 M and 3 M, signifying no correlation among the particles in magnetic fluids.