The variations of the microscopic morphology and oxides of metal uranium were in-situ investigated in the range of 20~400 ℃ by optical metalloscopy (OM), Raman spectroscopy(RS) and atomic force microscopy(AFM). Experiments were performed in ambient air, dry air and saturated vapor, respectively. The corrosion products and some inclusions were identified. In ambient air, uranium adsorbed oxygen, water and carbon dioxide, and subsequently reacted with them resulting in forming uranium dioxide, uranyl compounds, uranium carbonate etc. Contrastively in dry air, reactive corroded spots appeared at the beginning of heating and cumulated gradually, and generated pitting, with a simultaneous constituent transformation from uranium dioxde to triuranium octoxide. Meanwhile, uranium dioxde was heated at a low rate in saturated vapor, resulting in a conversion to triuranium octoxide at the temperature of 260~280 ℃.