Vanadium dioxide, which has been demonstrated as negative differential thermal emissivity materials for emissivity engineering application, is regarded as the most important and promising material for adaptive infrared camouflage. This study describes the synthesis and characterization of high purity, uniform VO2 nano powders by a single-step hydrothermal method and their thermal polymorphism. A dramatic phase transition of the monoclinic VO2 nanopowders to a tetragonal phase upon heating and the accompanying change in the IR emissivity are demonstrated. Importantly, we demonstrate the tenability of the infrared radiation intensity of VO2 nanopowders in both mid- and far-IR thermal atmospheric windows, for the first time. The structural transition is analyzed by variable temperature XRD and temperature-dependent Raman spectroscopy. Furthermore, an unexpected structural change in the same region of the sample was observed in a transmission electron microscope, which provides a direct mechanistic understanding of the infrared emissivity variation. Our study makes a significant contribution to the state-of-the-art in thermal camouflaging, because this study demonstrates that the VO2 nanopowders can be extremely promising camouflage materials in the area of adaptive infrared camouflage technology both in the far-infrared and mid-infrared regions and the material is suitable for substrates with large surface areas and/or complex morphologies.