This work reports structural properties of room temperature ferromagnetic Fe doped ZnO nanofibers (NFs). The NFs were obtained by electrospinning and calcination in air. The input atomic ratio of Fe to Zn ions was about 0.1. The structural characterization was performed by X-ray (XRD) examination, using the synchrotron radiation, and Energy-Filtered Transmission Electron Microscopy (EFTEM) analysis. Incorporating Fe ions into ZnO does not affect the crystal structure of the wurtzite host. No clear evidence of the second phase (Fe, FeO, Fe2O3 or ZnFe2O4) was detected. Diameters of crystals responsible for the magnetic properties ranged from 3 to 10 nm. We did not observe any precipitates of the different phase with diameters equal or larger than 1.5 nm. It implies that the magnetic signal comes from Fe ions built-in ZnO crystals. No other crystals (besides ZnO crystals) were observed in this range of sizes. We propose that the low activation energy for the nanocrystals growth in NFs allows a large amount of doped ions to be built-in the ZnO crystals.(C) 2013 Elsevier Ltd. All rights reserved.