The nature of the physical mechanisms responsible for the structural modification of the γ-Fe2O3 nanoparticles under laser irradiation has been investigated by Raman spectroscopy. In situ micro-Raman measurements were carried out on as-prepared γ-Fe2O3 nanoparticles about 4 nm in size as a function of laser power and on annealed γ-Fe2O3 particles. A baseline profile analysis clearly evidenced that the phase transition from maghemite into hematite is caused by local heating due to laser irradiation with an increase of grain size of nanoparticles. This increasing was clearly determined by X-ray diffraction from 4 nm in nanoparticles up to more than 177 nm beyond 900 °C in a polycrystalline state. Copyright © 2010 John Wiley & Sons, Ltd.