Energy harvester based on piezoelectric nanowires, i.e., nanogenerator, is an innovative device to harvest the ubiquitous mechanical energy from the environment. The development of the nanogenerator has already shown promising result in powering electronics in small scale and it can potentially advance the renewable energy technology in large scale. In this review paper, we introduce briefly the mechanical-electric energy conversion process of piezoelectric materials. We then take an overview of recent research efforts on the nanogenerator based on wurtzite material, perovskite material, and piezoelectric polymer. The improvement of power output is critical and different scale-up techniques to integrate more functional units in a single nanogenerator are discussed. The integration of the nanogenerator with other electronic devices, such as pH sensor, UV sensor, LED, and LCD, is very encouraging. The research of nanogenerator may lead to a self-powered system, and the challenges are also noticed.