CuO nanosheets and dendrites have been synthesized by a microwave hydrothermal method. The synthesized CuO nanostructures were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and transition electron microscopy (TEM). The formation process was discussed, which revealed that the disassembly of sheet-built CuO dendrites contributed to the formation of discrete CuO nanosheets under microwave hydrothermal conditions. However, such a disassembly process was inhibited by an ultrasonic heat pretreatment before the microwave hydrothermal treatment. The synthesized CuO nanostructures were active toward the thermal decomposition of ammonium perchlorate (AP) and allowed the decomposition temperature of AP to decrease. Furthermore, the synthesized CuO nanostructures promoted the photodegradation of rhodamine B and methyl orange.