Mechanical energy sources are abundant in our living environment, such as body motion, vehicle transportation, engine vibrations and breezy wind, which have been underestimated in many cases. They could be converted into electrical energy and utilized for many purposes, including driving small electronic devices or even constructing an integrated system operated without bulky batteries and power cables. Many progresses have been made recently in the mechanical energy harvesting technology based on piezoelectric nanogenerators (PENGs). By introducing a new sandwich structure design, high performance PENGs can be achieved through very simple fabrication process with good mechanical stability by utilizing ZnO nanowires (NWs). By further optimizing the nanomaterials׳ properties and device structure, the PENG׳s open circuit (OC) voltage can be elevated to over 37V. Two important applications of this technology are that the nanogenerator can be used as a sustainable power source for self-powered system and can worked as active sensors. Several demonstrations are reviewed here. Finally, perspectives of this mechanical energy harvesting technology are discussed. Co-operation with power management circuit, capability of integrating with a system, and low cost large-scale manufacturing processing are suggested to be the key points toward commercialization of PENGs.