Uniform-sized MnO nanospheres and nanorods were fabricated by the thermal decomposition of Mn-surfactant complexes. The particle sizes of the nanospheres were varied. from 5 to 40 nm by changing the surfactant. The shape of the particles could be controlled by varying the experimental conditions. The synthesized MnO nanorods have diameters ranging from 7 to 10 nm, and lengths ranging from 30 to 140 nm. Structural characterization using X-ray powder diffraction, X-ray absorption spectroscopy, and high-resolution transmission electron microscopy revealed that the nanoparticles are core/shell structures with a MnO core and a thin Mn3O4 shell. The MnO nanoparticles exhibited very interesting magnetic properties. For example, the MnO nanorods with dimensions of 7 nm x 33 nm showed two blocking temperatures at 35 and 280 K, respectively.