The formation mechanism of highly pure H 2 V 3 O 8 single-crystal nanobelts is clarified in a hydrothermal synthesis process with a specially designed precursor solution containing V 5+ and V 4+ in a fixed ratio of 2/1. This specially designed precursor solution provides an additional merit for the rapid fabrication of highly pure H 2 V 3 O 8 nanobelts through a simple hydrothermal route. During the hydrothermal synthesis process, V 5+ species initially reacts with some V 4+ to form a metastable, whisker-like V 10 O 24 AE nH 2 O (n < 12). The V 5+ species dissolved from the whisker-like V 10 O 24 AE nH 2 O reacts continuously with residual V 4+ ions in the precursor solution to form seeds of H 2 V 3 O 8 single-crystals. The anisotropic growth of H 2 V 3 O 8 single-crystal nanobelts with length > 10 lm and width between 50 and 150 nm occurs with prolonging the hydrothermal time. Finally, highly pure H 2 V 3 O 8 single-crystal nanobelts are obtained when the hydrothermal time reaches 4 h. The textures of vanadium oxides prepared at different hydrothermal times are systematically compared through X-ray diffraction, transmission electron microscopic and X-ray photoelectron spectroscopic analyses to clarify the synthesis mechanism of H 2 V 3 O 8 single-crystal nanobelts.