In this paper, we report a rapid synthesis of piezoelectric ZnO-nanostructures and fabrication of the nanostructures-based power-generators demonstrating an energy conversion from an environmental mechanical/ultrasonic energy to an electrical energy. The ZnO nanostructures are grown on a silicon wafer by a modified chemical solution method (CSD, chemical-solution-deposition) with a two-step thermal-oxidation approach. The synthesis process can be completed within 1h. By varying the mixture-ratio of Zn micro-particles in an oxalic acid solution with 0.75mol/l concentration in the CSD process, the growth mechanism is well-controlled to synthesize three different types of ZnO-nanostructures (i.e., dandelion-like nanostructures, columnar nanostructures, and nanowires). Furthermore, through oxidizing at different temperatures in the thermal-oxidation process, the featured geometry of the nanostructures (e.g., the length and diameter of a nanowire) is modified. The geometry, size, morphology, crystallization, and material phase of the modified nanostructures are characterized by scanning electron microscopy and X-ray diffraction. Finally, the nanostructures are used to fabricate several micro power-generators. Through the piezoelectric effect, a maximum current density output of 0.28μAcm-2 generated by a power-generator under an ultrasonic wave is observed.