Iron oxide (Fe2O3) has four crystal structures: gamma-, epsilon-, beta-, and alpha-Fe2O3. Until now, routes of the phase transformations among the four Fe2O3 phases have not been clarified because a systematic synthesis that yields all four Fe2O3 phases has yet to be reported. Herein we report the synthesis of a series of Fe2O3 nanoparticles using mesoporous SiO2. The crystal structures of the Fe2O3 nanoparticles change in the order of gamma-Fe2O3 -> epsilon-Fe2O3 -> beta-Fe2O3 -> alpha-Fe2O3 as the particle size increases. Threshold sizes were estimated as gamma -> epsilon at 8 nm, epsilon -> beta at 30 nm, and beta -> alpha at 50 nm in the synthesis using FeSO4 as a precursor. The phase transformations among the four Fe2O3 phases have been observed for the first time.