This paper presents the design and fabrication of a piezoelectric micro power generator to be used as a power source for a wireless sensor node. The device scavenges environmental vibrations and converts it into electrical power through a piezoelectric transduction. The device, that measures 2x2x0.5mm is a resonant structure composed of a seismic mass attached to a cantilever. The device has been built with a MEMS process. The paper presents modelling and simulation of devices using either AlN or PZT piezoelectric layer. The measurements made on a first prototype using an AlN layer have shown a power output of 38nW for 0.5g of excitation at frequency of 204Hz.