Highly uniform rod-like beta-MnO2 microcrystals have been synthesized via a urea mediated hydrothermal method. These monocrystalline products are of tetragonal zircon structure with growth direction along [110]. The metal-insulator (M-I) transition displayed from resistivity shifts to a higher temperature, which is induced by the existence of oxygen defects. Congruously with the M-I transition temperature, the magnetoresistance (MR) ratio reaches an abrupt peak and then slowly increases with temperature, exceeding 50% at room temperature under 5.0 T. This numerously enhanced positive room temperature MR can be mainly attributed to the formation of oxygen defects during the hydrothermal process. (c) 2006 Elsevier B.V. All rights reserved.