Magnetization expression of weak interacting (mH/kT = 1-1.5) magnetite nanoparticles mediums was determined. Experimental and calculated magnetization curves, initial magnetic susceptibility of magnetite nanoparticle mediums were compared. It is showed are agree. Additional term of the magnetization is sufficient at weak and middle magnetic fields and at large magnetic fields it leads to zero. The maximum value of additional term is determined as mH/kT = 1.566 and it is linear function of magnetic moments and parabolic function of concentration of magnetite.