This paper deals with the design and the vibration analysis of a small-size (30 kVA) cogeneration unit, which can produce electric and thermal energy both for domestic and industrial appliances. To optimize the machine unsatisfactory dynamic behaviour (vibrations and noise), a discrete system approach was at first considered, also employing the design of experiments (DOE) technique to easily attain the best solution, then FEM analysis and measures with accelerometers on a prototype were performed. The solutions obtained allowed a significant improvement of the cogenerator behaviour, without adding any costs, being the final structure of the machine almost equal to the original version.