We have successfully synthesized monodisperse alpha-Fe2O3 nano/micro-cubes with sizes ranging from 40 to 840 nm. High-temperature magnetic measurements under a high vacuum were used to systematically study the size dependence of phase transition from alpha-Fe2O3 to Fe3O4. Through a series of characterizations, we concluded that the different sizes of particles have a significant impact on their thermal stability. The data shows a clear correlation between the onset temperature of the phase transition and the size of particles. This phenomenon can be well explained in terms of the dependence of enthalpy on surface area. Moreover, Morin transition of the alpha-Fe2O3 phase also have been measured for our products at the nanoscale. The result reflects that the Morin transition temperature decreases with decreasing the size of the particle, which supports finite-size scaling law according to previous findings. Our conclusions provide an important insight into many other size-dependent structural phase transitions. (C) 2016 Published by Elsevier B.V.