Fe2O3 is widely used in the field of magnetism, and its magnetic properties are affected by particle size and crystalline form. Therefore, it is of great importance to study the effects of particle size on the structural transitions and magnetic properties. In this paper, accurate relations were derived to describe size dependence of the integral enthalpy and entropy of structural transitions of nanocrystals, and the influence mechanism of particle size on the thermodynamic properties of structural transitions is discussed. Different sizes of nano-Fe2O3 were prepared, and differential scanning calorimetry (DSC) was used to obtain the temperature, enthalpy, and entropy of the structural transition from maghemite (gamma-Fe2O3) to hematite (alpha-Fe2O3). The magnetic properties of nano-gamma-Fe2O3 were determined on the Magnetic Property Measurement System. We found that the thermodynamic properties of the structural transition decrease as particle size decreases and vary linearly with the reciprocal of particle size within the range of experimental particle size. In addition, it is found that the magnetic properties of nano-Fe2O3 also show an obvious size dependency. As particle size increases, the saturated magnetization increases sharply and remains basically unchanged when reaches 60 nm; at the minimum particle size (19.3 nm), both the coercivity and the remanence approach zero and increase sharply with increasing particle size and reach the maximums at about 60 nm and then decrease. These findings improve our understanding of the effects of particle size on the thermodynamic properties of structural transitions and magnetic properties and provide a scientific guidance for the design, preparation, study, and application of Fe2O3 and related