The design, fabrication, performance simulation and characterization of a piezoelectric zinc oxide (ZnO) nanowire (NW) micro-energy harvester integrated with a microelectromechanical proof-mass/spring system are presented. Low temperature hydrothermal growth of vertically aligned nanowires together with MEMS surface micromachining and UV-LIGA (Lithographie, Galvanoformung, Abformung) processes were utilized to achieve efficient coupling of environmental vibrations to the nano energy harvester. The NWs were connected in series through the membrane/proof-mass structure to achieve higher output voltage. A novel polyimide fill-in and etch-back technique was employed to embed the NW tips into the membrane, such that the NWs were compressed and stretched by the membrane/proof-mass structure in response to the external vibrations. Peak power of 70pW was observed across an optimal 7MΩ load when the nano energy harvester was excited with an acceleration of 1.4g at 40Hz, corresponding to 22 mW/cm3 volume-normalized power output. The measured voltage between the top and the bottom electrode was 21.4 mV for this case. The output frequency spectral density with 40Hz excitation and output voltage with varying excitation frequency are also discussed.