MoO3 nanobelts with good crystallinity were synthesized from molybdic acid, hydrogen peroxide and nitric acid through a microwave-assisted hydrothermal process. The synthesis method was simple, rapid, low-cost, safe, and also suitable for large-scale fabrication. The morphology, structure and chemical state of the product were characterized by SEM, TEM, XRD, XPS and FT-IR, respectively. Furthermore, a possible growth mechanism of the nanobelts was presented based on the experimental observations.