Electret or permanent electrical charge is used as a part of mechano-electric energy conversion system for vibrational energy harvesting device. Microscopic capacitors are produced by silicon micromachining or MEMS (microelectromechanical systems) fabrication process, and the surfaces are turned into impurity-rich silicon oxide, which is later processed to make an electret by displacing the impurity ions by the same mechanism as the anodic bonding. The built-in potential of the electret is used to produce electrical current through the electrostatic induction when the movable electrode is periodically shaken by the external vibrations. In this article, the fabrication processes for the electret as well as the MEMS vibrational energy harvester are discussed. The fundamental characteristics of the energy harvesters are reported, along with a demonstration result as an autonomous powerpack for an IoT (internet-of-things) type wireless sensor node.