A low-temperature hydrothermal method was developed to synthesize urchinlike gamma-MnO2 nanostructures. Time-dependent evolutions of morphology and crystallinity were investigated to explore the growth mechanism of the gamma-MnO2 urchins. The results show that the growth process of the gamma-MnO2 urchins occurs in two main stages, which are the generation of gamma-MnO2 microspheres and the following epitaxial growth of gamma-MnO2 nanoneedles on the surface of the initial microspheres. Microwave absorption properties of the urchinlike gamma-MnO2 nanostructures were studied in terms of complex permittivity and permeability. An effective absorption bandwidth (reflection loss lower than -10 dB) of 8.8 GHz was achieved from the gamma-MnO2/paraffin wax composite. (C) 2011 Elsevier B.V. All rights reserved.