Magnetic gels are a new class of soft polymer materials with the properties controlled by magnetic field. Method of preparation of magnetite nanoparticles in a matrix of poly(acrylamide) gel was developed. Synthesis of magnetic spinel iron oxide Fe3O4 in the matrix of poly(acrylamide) gel have been performed through coprecipitation of Fe(II) and Fe(III) in alkaline media. Effect of the cross-links density, of the concentration of polymer and salts in the swollen networks during the preparation on the composition, structure and properties of magnetic gels have been studied by electronography, TEM and small angle X-ray scattering (SAXS). It was shown that the crystalline lattice of the particles is determined as a magnetite. The average size of the particles, d calculated from the half-width of the diffraction peaks is of the order of 10 nm. The obtained data demonstrate a marked difference in the size distribution of magnetic particles in the gels with different structure. The applied magnetic field can affect the size and the shape of the particles formed in the magnetic gel.