Piezoelectric converters designed for harvesting energy from mechanical vibrations have been fabricated by micromachining technologies. They are characterized by applying a sinusoidal oscillation as mechanical input and by using a resistive load to measure the output power of the system. A maximum output power of 40μW has been measured for a PZT based harvester excited by an input vibration having a frequency of 1.8kHz and an amplitude of 180nm. Preliminary experimental results on AlN based devices are also presented. A model capable of estimating the output power as a function of material parameters and device dimensions has been developed. Theoretical estimations have been compared with experimental results.