NiO nanorods and microflowers were successfully achieved via a hydrothermal reaction without any surfactant and then a post heat treatment process. Characterization methods, such as X-ray diffraction, transmission electron microscopy and scanning electron microscopy, were employed to systematically investigate the mechanism for the morphology evolution, as well as the transformation of phase structure. Furthermore, the physicochemical properties of NiO with different morphologies were also examined and it was found that these properties were pretty sensitive to the morphology. Compared with the nanorods, the flower-like NiO nanostructures showed a great enhancement of the ferromagnetic properties. The catalytic activities of NiO with different morphologies were investigated through the thermal decomposition of ammonium perchlorate (AP). AP exhibited relatively low decomposition temperature and fast reaction rate in the catalyzed formations making NiO with microflower structure an effective catalyst. This study provided a facile way to control the morphology of NiO nanostructures, which also indicates the potential application of the nanostructures, with the modified magnetic properties, in modern science and technology.