Iron nanospheres, nanoflakes and nanofibers were synthesized via a simple pyrolysis method. When the pyrolysis temperature increased from 523 to 623 K and the flow rate of Ar carrier gas maintained at 100 sccm, the as-prepared iron nanocrystals showed a morphology evolution from isotropic nanospheres to isotropic nanofibers. The phase structures and morphologies of the composite were characterized by X-ray diffraction and scanning electron microscopy. The complex permittivity (epsilon' - j epsilon aEuro(3)) and permeability (mu' - j mu aEuro(3)) of these composites were measured using the transmission/refection coaxial line method in the frequency range of 1-18 GHz by a vector network analyzer. The iron nanofibers exhibited superior microwave absorbing properties compared to iron nanoparticles and nanoflakes. The optimal reflection loss (RL) reached -17.8 dB at 9.9 GHz with a layer thickness of 2.0 mm. The RL below -10 dB can be obtained in the frequency range of 7.3-11.7 GHz. Considering the low cost and high efficiency; the iron nanofibers are favorable for application as microwave absorber.