In this study, yarn-ball-like tungsten oxide microspheres were synthesized using a solvothermal method. Highly oriented (020) plane yarn-ball-like tungsten oxide microspheres were assembled using tungsten oxide nanowires approximately 5-15 nm in diameter and tens of microns in length. After the postannealing treatment, the morphologies of the tungsten nanowires transformed into aggregated nanoparticles approximately 20-80 nm in diameter. The morphologies and structures of the as-synthesized yarn-ball-like tungsten oxide microspheres and aggregated nanoparticles treated postannealing were characterized using field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FT-IR), and Raman spectroscopy. This study proposes the possible growth mechanism of yarn-ball-like tungsten oxide microspheres assisted by solvent molecules. (C) 2014 Elsevier B.V. All rights reserved.