The kinetic of the phase transition of maghemite to hematite was measured with the in situ x-ray powder diffraction. It could be shown that activation energy is dependent on the crystal size of the maghemite. With decreasing crystal size the energy is getting smaller. This result could be explained with the change of the geometric properties during the transition and the resulting change of the ratios of surface energies and volumetric energies. With a geometric model the change of the free enthalpy during the phase transition could be calculated for different maghemite crystal sizes. The characterisation of the samples was done by x-ray powder diffraction, x-ray absorption spectroscopy and transmission electron microscopy.