An effect of activation with nanosized particles Fe3O4 on structural and magnetic properties of zinc-tellurite glasses 20·ZnO-(80-x)·TeO2-x·Fe3O4 (x=0, 1, 3, 7), prepared with melt-quenching method, have been investigated. Some functional properties of the glasses were experimentally investigated with X-ray diffraction, Energy dispersive and Raman spectroscopies, and SQUID magnetometry. The values of the average crystallite sizes calculated with Scherer and Williamson-Hall methods were comparable with the sizes of magnetically active clusters reconstructed from the results of processing of the magnetic field dependences of the values of specific magnetizations, indicating their effect on the change in the magnetic properties of activated glasses. A partial phase transformation, caused by oxidation of Fe2+ into Fe3+ ions, of ferrimagnetic magnetite Fe3O4 into paramagnetic maghemite γ-Fe2O3 has been revealed. Keywords: glass, glass-ceramics, nanoparticle, magnetite, magnetism.