In this contribution, monodisperse porous hollow bi-phase γ-/α-Fe2O3nanoparticles were successfully fabricated based on hard-template method with using carbon colloids as sacrificial templates. A new concept of assembling one kind of metal oxide with different crystalline structures into a single shell was presented for the first time. The critical procedure of coating carbon cores with a uniform layer of oxide was performed in CO2-expanded ethanol, which is a versatile way to produce high-quality hollow oxidenanoparticles. The formation of the novel bi-phase shell was achieved through combining the reduction ability of carbon cores under inert calcination atmosphere and the unique chemical composition of intermediate-shell formed in CO2-expanded ethanol. The porous hollow γ-/α-Fe2O3nanoparticles with an average diameter of 99 nm not only possess combined properties of γ-Fe2O3 and α-Fe2O3, but also have a large specific surface area of 93.7 m2 g−1 and a high pore volume of 1.056 cm3 g−1, enabling them to have widespread applications in sensors, catalysis, magnetic and electrochemical areas, etc. Herein, such hollow bi-phase γ-/α-Fe2O3nanoparticles were utilized to prepare a sensor device, and intriguingly it shows higher sensitivity and selectivity to ethanol than γ-Fe2O3 powders and many other porous α-Fe2O3 materials reported recently. The probable sensor mechanism of hollow γ-/α-Fe2O3nanoparticles was discussed in detail.