Fe-Mn oxide (Mn_xFe_1-xO) nanoplates were prepd. by the thermal decompn. method. Irregular development of cryst. phases was obsd. with the increase of annealing temp. Magnetic properties are in accordance with their resp. cryst. phases, and the selective magnetic evolution from their rich magnetism of Mn_xFe_1-xO and MnFe₂O₄ is achieved by controlling the annealing conditions. Rock-salt structure of Mn_xFe_1-xO (space group Fm̅3m) is obsd. in as-synthesized nanoplates, while MnFe₂O₄ and Mn_xFe_1-xO with significant magnetic interactions between them are obsd. at 380°. In nanoplates annealed at 450°, soft ferrites of Mn0.48Fe_2.52O₄ with Mn_xFe_1-xO are obsd. It is assumed that the differential and early development of cryst. phase of Mn_xFe_1-xO and the inhomogeneous cation mixing between Mn and Fe cause this rather extraordinary magnetic development. In particular, the prone nature of divalent metal oxides to cation vacancy and the prolonged annealing time of 15 h which enables ordering also probably contribute to these irregularities.