This paper describes a vapor-phase, precursor method that generates MgO nanowires at relatively low temperatures (800-900degreesC) as compared to previous approaches (greater than or equal to1200degreesC) that directly use MgO as the source material. Magnesium diboride (MgB2) powders were used as the precursor, which slowly decomposed into Mg and MgB4 under a constant flow of argon gas at temperatures as low as 700degreesC. The Mg vapor subsequently reacted with trace O-2 contained in the reaction system and formed MgO vapor. Under appropriate conditions, the MgO vapor could reach supersaturation, condense onto the surface of a solid substrate that was placed on top of the precursor powders, and grow into highly anisotropic nanostructures. Depending on the distance between the MgB2 source and the location on the solid substrate, a gradient in the MgO concentration existed. As a result, MgO nanostructures with different morphologies were formed on the solid support, including whiskers, tapered nanowires whose diameters were continuously reduced from similar to200 to 20-30 nm over a distance of similar to50 mum, and nanowires having uniform diameters (15-20 nm) and lengths up to similar to30 mum. When a mixture of dihydrogen and argon gases was used as the reaction atmosphere, MgO nanowires with a uniform cross-section of similar to150 nm in diameter could grow several millimeters long without branching.