An effective technique for obtaining near-theoretical density niobium oxide by utilizing the critical nuclei size-effect is reported. Dense nanostructured niobium oxide with more than 99% of the theoretical maximum and an average grain size of less than 90 nm was produced by sintering during a metastable-to-stable phase transformation at 650 degrees C. The tendency of the particulate system to reach the critical nuclei size by reducing the surface energy through the replacement of solid-gas interface by solid-solid interface (densification process) is one of the main reasons for this excellent sintering behavior.