Single crystal manganese oxyhydroxide (MnOOH) nanorods were successfully prepared without extra template or surfactant employing a simple hydrothermal route based on the redox between potassium permanganate (KMnO4) and N-methyl pyrrolidone (NMP). By adjusting the reaction temperature, reaction time, and the volume ratio of H2O/NMP, MnOOH nanorods with lengths up to 20 μm and square cross-sections with edge lengths in the range of 50-400 nm could be facilely prepared in high yield. The as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and Fourier transformed infrared spectroscopy (FTIR). The formation mechanism of MnOOH nanorods was preliminarily discussed based on the Ostwald's ripening process. Furthermore, Mn2O3 nanorods were obtained by calcination of MnOOH nanorods.