A simple one-pot synthesis of RuO 2 nanoparticles by the thermal decomposition of ruthenium nitrosyl nitrate [Ru(NO)(NO 3 ) 3 ] up to 800°C was investigated. The RuO 2 phase was characterized by XRD, SAXS, FE SEM/EDS, Raman, DTA/TGA and FT-IR techniques. Broadening of prominent diffraction lines (110), (101) and (211) was used to estimate nanocrystallite sizes in RuO 2 particles. FE SEM showed the formation of RuO 2 plates at 400°C which consisted of RuO 2 nanoparticles of about 15–25nm in size. At 800°C RuO 2 nanoparticles showed the sintering effect and some of them increased in size to about 200nm. Raman and FT-IR spectra consolidated the findings. Moreover, DTA curves showed the decomposition and release of NO and NO 3 − groups to be a stepwise process.