alpha-MnO2 nanowires and beta-mnO(2) nanorods with high crystallinity were obtained successfully by a hydrothermal method at 160 degrees C for different reaction times. As-synthesized samples were analyzed by Xray diffraction (XRD) spectra, scanning electronic microscopy (SEM), transmission electronic microscopy (TEM), and high-resolution TEM (HRTEM). The results show that the samples changed from alpha-MnO2 nanowires with diameters about 50 nm and lengths 2-4 mu m to beta-MnO2 microrods with diameters 2-5 mu m and lengths 5-10 mu m under the reaction time prolonged from 2 to 48 h. The dielectric properties and microwave absorbing properties of the as-prepared samples were examined in the frequency range of 2-12 GHz. The samples under the reaction time 2 and 24h exhibited the most dielectric loss tangent values and microwave absorption performances. The effective absorption bandwidth which is lower than -10 dB is 3.2 and 1.4 GHz, respectively. The results show that the morphology is important to the microwave absorption characteristics of manganese dioxide. It is also found that MnO2 is a kind of materials with both dielectric and diamagnetic loss, and the dielectric loss results from space charge polarization, which plays the dominant role in the total loss. (C) 2010 Elsevier B.V. All rights reserved.