Oxide nanostructures can be prepared by various kinds of methods like thermal evaporation, vapor phase epitaxy, chemical synthesis, etc. Among various synthesis methods, hydrothermal method is of interest because it is simple and environmentally friendly. Also, it is performed at relatively low temperatures (<150C), which is a significant advantage for device fabrication, in particular for hybrid organic/inorganic devices on flexible plastic substrates. In this work, nanostructures of various kinds of oxides: ZnO, CuO, NiO, and Ga2O3 were fabricated from aqueous solutions of the respective metal nitrate hydrate and hexamethylenetetramine. The obtained nanostructures were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The influence of solution concentration, synthesis temperature, and solution pH value on the obtained morphology and properties was investigated. Growth mechanisms and the factors affecting the structures of the investigated metal oxide nanostructures are discussed.