Morphology transformation and crystal growth strategies of metal oxide semiconductors are still extensively studied in material science recently, because the morphology and crystallinity significantly affect the physicochemical characteristics of metal oxide nanomaterials. However, understanding the morphology changes of α -MoO 3 induced by annealing is still a challenge. Herein, the nanostructure transition of α -MoO 3 induced by the annealing temperature is carefully investigated via the XRD and SEM methods. It can be found that crystallization is highly dependent on the annealing temperature. Interestingly, the MoO 3 nanoflowers can change into nanosheets at 500 °C. Afterward, the nanosheets turned into microrods with the increase in annealing temperature due to the continuous growth of MoO 3 crystal. On the other hand, the sensing performances of various MoO 3 nanostructures are studied toward ethanol gas. Compared to the MoO 3 nanoflowers and microrods, the MoO 3 nanosheets-based sensor exhibits superior sensing performance to ethanol, and the maximum response value is 8.06.