Uniform single-crystalline CoS 2 nano-octahedrons have been prepared in high yield by a simple hydrothermal process. The octahedral structures exhibit a high geometric symmetry with smooth surfaces and the mean edge length is ~120nm. X-ray diffraction, scanning electron microscopy, transmission electron microscopy were used to characterize the samples. Contrast experiments indicate that the capping agent poly(vinyl pyrrolidone) (PVP) and the complexing agent sodium citrate play important roles for the formation of octahedral CoS 2 nanostructures. A possible formation mechanism was proposed based on the evolution of this morphology as a function of hydrothermal time. Additionally, the magnetic properties of the CoS 2 nano-octahedrons were discussed in detail.