Piezoelectric nanogenerator (PENG) has been reported as a green energy solution that generates electrical power from mechanical deformations resulted in the environment. Here, a novel design concept of PENG is reported based on mechanical metamaterials (MM). Plate-like MM with piezoelectric materials is designed in a sliding cell such that low frequency excitations causes structural instability (i.e., postbuckling) of the piezo-MM and thus, the piezo layer is triggered to generate electrical power. In this process, the environmental input (