Magnesium-manganese ferrite nanopowders (Mg_xMn_1-xFe₂O₄, x=0.0 up to 1 with step 0.2) were prepared by coprecipitation method. The as-prepared samples were pressed with hydrolic press to form a pellet and were sintered in 900, 1050 and 1250˚C. Scanning Tunneling Microscope (STM) images showed the particle size of powders about 17 nm. The X-ray patterns confirmed the formation of cubic single phase spinel structure in samples sintered at 1250˚C. Substituting Mg²⁺ with Mn²⁺ in these samples, the lattice parameter decreased from 8.49 to 8.35Å and magnetization saturation decreased from 74.7 to 21.2emu/g. Also, coercity (H_C ) increased from 5 to 23Oe and Curie temperature (T_C ) increased from 269 to 392˚C. Samples with x= 0.2, 0.4, 0.6 sintered at 1250 ˚C, because of their magnetic properties, can be recommended for hyperthermia applications and for phase shifters.