Nanotechnology and nanoscience are experiencing rapid development in the last decade. Intensive research has been carried out on nanostructures synthesis and nanodevices fabrication. Due to its small size, a nanodevice usually requires an extremely small power to operate. However, to make the novel nanodevice work, an external power source is normally needed, which can either be a battery or a power source, thus, the size of the battery is usually much larger than that of the device and its life time is limited. It is highly desired to have a nanoscale size power source that harvests its energy from the environment so that it works independently and wirelessly to provide power to the nanodevices. This dissertation provides a solid solution to this dilemma based on nanotechnology. Starting from the synthesis of well aligned ZnO nanowire arrays on different substrates, an innovative method is presented first to measure the mechanical property of the as-synthesized ZnO nanowire arrays by using AFM without destroying and manipulating the sample. This technique is then extended to converte mechanical energy into electricity by scanning the nanowire arrays using a AFM tip in contact mode. Due to the unique semiconducting and piezoelectric dual properties of ZnO, mechanical energy is converted into electricity and is effectively output. This is the invention of the piezoelectric nanogenerator. Then, by replacing AFM tips using a zigzag top electrode, the first prototype direct-cirrent nanogenerator driven by ultrasonic wave has been fabricated. Further investigations have also been carried out about the effect of ZnO carrier density on the output power, and the power generating property of oligomer functionalized ZnO nanowires. This desertation established the fundamental mechanism for the nanogenerator, and it provides a new path towards self-powered nanosystems, which has key applications in in-vivo biosensing, MEMS, environmental mornitoring, defence technology and even personal electronics.