MoO3 nanostructures with different morphologies, such as helical nanosheets, crosslike nanoflowers, and nanobelts, have been synthesized on a large scale by an environmentally friendly chemical route. The evolution process from helical nanosheets to crosslike nanoflowers to nanobelts is observed for the first time. The influences of reaction time and the molar ratio of molybdenum and H2O2 on the morphologies of MoO3 nanostructures have been investigated. The synthetic process is environmentally friendly and may be extended to synthesize nanostructures of other metal (W, Ti, and Cr) oxides.