Solid-state reaction method is a common and effective technique to synthesize ferrites. This work investigated the phase transformation of MnO2 and Fe2O3 system roasted at 500-1400 degrees C in air atmosphere to understand the formation process of manganese ferrite. The results showed that the formation of manganese ferrite (MnxFe3-xO4) was derived from the reaction between Fe2O3 and Mn3O4 (the decomposition product of MnO2). Below 900 degrees C, MnO2 firstly decomposed to Mn2O3 and then to Mn3O4, and Fe2O3 was seldom reacted with Mn2O3 and Mn3O4. When the temperature went up to 1000 degrees C, Fe2O3 easily reacted with Mn3O4 to generate manganese ferrite. The reaction degree was enhanced dramatically with the rising of temperature. Moreover, the x value in the MnxFe3-xO4 increased from 0 to 1 from 900 degrees C to 1400 degrees C. In other words, the higher the temperature was, the closer the MnxFe3-xO4 was to MnFe2O4. Thermodynamic analysis of MnO2-Fe2O3 system under different O-2 partial pressures was carried out to further explain the formation mechanism. (C) 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.