Fe-Mn oxide (MnxFe1-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 MnxFe1-xO and MnFe2O4 is achieved by controlling the annealing conditions. Rock-salt structure of MnxFe1-xO (space group Fm̅3m) is obsd. in as-synthesized nanoplates, while MnFe2O4 and MnxFe1-xO with significant magnetic interactions between them are obsd. at 380°. In nanoplates annealed at 450°, soft ferrites of Mn0.48Fe2.52O4 with MnxFe1-xO are obsd. It is assumed that the differential and early development of cryst. phase of MnxFe1-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.